Re: [Xen-devel] Unifying x86_64 / Xen init paths and reading hardware_subarch early

[Konrad Rzeszutek Wilk]

On Fri, Jan 15, 2016 at 03:47:25PM -0800, Andy Lutomirski wrote:
> On Fri, Jan 15, 2016 at 2:08 PM, Luis R. Rodriguez  wrote:
> > I will be respinning the generic Linux linker table solution [0] soon
> > based on hpa's feedback again now that I'm back from vacation. As I do
> > that though I wanted to highlight a feature I'm throwing into the
> > linker table solution which I am not sure many have paid close
> > attention to but I think is important to Xen. I'm making use of the
> > zero page hardware_subarch to enable us to detect if we're a specific
> > hypervisor solution *as early as is possible*. This has a few
> > implications, short term it is designed to provides a proactive
> > technical solution to bugs such as the cr4 shadow crash (see
> > 5054daa285beaf706f051fbd395dc36c9f0f907f) and ensure that *new* x86
> > features get a proper Xen implementation proactively *or* at the very
> > least get annotated as unsupported properly, instead of having them
> > crash and later finding out. A valid example here is Kasan, which to
> > this day lacks proper Xen support. In the future, if the generic
> > linker table solution gets merged, it would mean developers would have
> > to *think* about if they support Xen or not at development time. It
> > does this in a not-disruptive way to Xen / x86_64 but most
> > *importantly* it does not extend pvops! This should avoid issues in
> > cases of developer / maintainer bandwidth, should some new features be
> > pushed onto Linux for x86_64 but a respective Xen solution is not
> > addressed, and that was not caught early in patch review, such as with
> > Kasan.
> >
> > [0] 
> > https://lkml.kernel.org/r/1450217797-19295-1-git-send-email-mcg...@do-not-panic.com
> >
> > Two things I'd like to request a bit of help with and review / 
> > consideration:
> >
> > 1) I'd like some advice on a curious problem I've stumbled on. I'd
> > like to access hardware_subarch super early, and in my review with at
> > least two x86 folks this *should* work:
> >
> > diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
> > index c913b7eb5056..9168842821c8 100644
> > --- a/arch/x86/kernel/head64.c
> > +++ b/arch/x86/kernel/head64.c
> > @@ -141,6 +141,7 @@ static void __init copy_bootdata(char *real_mode_data)
> >
> >  asmlinkage __visible void __init x86_64_start_kernel(char * real_mode_data)
> >  {
> > + struct boot_params *params = (struct boot_params *)__va(real_mode_data);
> >   int i;
> 
> This is a mess :-p
> 
> If you want to access real_mode_data before load_idt, you'll need to do:
> 
> for (i = 0; i < sizeof(boot_params); i += 4096)
> early_make_pgtable((unsigned long)params + i);
> 
> Of course, it's entirely possible that that will blow up if you try to
> do it on Xen.

That real_mode should have already been setup by Xen by the time you
call this code. (I hope).

> 
> I think this would all be easier to understand if you try to separate
> out the ideas of linker tables from the idea of rearranging early
> init.  AFAICT the linker table thing is just an implementation detail.
> 
> If I understand right, you're trying to unify the Xen and native
> startup as much as possible.  Why not add little shims, though?
> Create a new start_kernel_common(int subarch, ...) where subarch
> indicates native vs Xen and have its callers tell it which mode it's
> in?
> 
> --Andy

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Re: [Xen-devel] Unifying x86_64 / Xen init paths and reading hardware_subarch early

[Borislav Petkov]

On Fri, Jan 15, 2016 at 05:39:05PM -0800, Luis R. Rodriguez wrote:
> On Fri, Jan 15, 2016 at 4:43 PM, Luis R. Rodriguez  wrote:
> >> for (i = 0; i < sizeof(boot_params); i += 4096)
> >> early_make_pgtable((unsigned long)params + i);
> >
> >  I'll give this a shot.
> 
> Thanks again for this! It seems to let this boot now! But it does not
> seem to provided the right value. If I use the qemu debug patch as I
> listed before to set this to 5 for kvm, and boot it doesn't come up.
> This can be tested with the qemu debug patch + this debug kernel patch
> which prints it out and resets it from what it finds early.
> 
> If you comment out the boot_params.hdr.hardware_subarch =
> my_hardware_subarch; assignment we get the right value from the
> copy_bootdata() work. I use my_hardware_subarch just as a quick hack
> to test and cache the value early code gets but that I can't print
> early on.

You can always do stupid debug loops:

while (subarch == )
rep_nop();

and when your guest stops booting and gdb points you here, then you know
what's going on. You can then dump interesting stuff too from gdb.

-- 
Regards/Gruss,
Boris.

SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 
(AG Nürnberg)
-- 

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Re: [Xen-devel] Unifying x86_64 / Xen init paths and reading hardware_subarch early

[Luis R. Rodriguez]

On Fri, Jan 15, 2016 at 03:47:25PM -0800, Andy Lutomirski wrote:
> On Fri, Jan 15, 2016 at 2:08 PM, Luis R. Rodriguez  wrote:
> > I will be respinning the generic Linux linker table solution [0] soon
> > based on hpa's feedback again now that I'm back from vacation. As I do
> > that though I wanted to highlight a feature I'm throwing into the
> > linker table solution which I am not sure many have paid close
> > attention to but I think is important to Xen. I'm making use of the
> > zero page hardware_subarch to enable us to detect if we're a specific
> > hypervisor solution *as early as is possible*. This has a few
> > implications, short term it is designed to provides a proactive
> > technical solution to bugs such as the cr4 shadow crash (see
> > 5054daa285beaf706f051fbd395dc36c9f0f907f) and ensure that *new* x86
> > features get a proper Xen implementation proactively *or* at the very
> > least get annotated as unsupported properly, instead of having them
> > crash and later finding out. A valid example here is Kasan, which to
> > this day lacks proper Xen support. In the future, if the generic
> > linker table solution gets merged, it would mean developers would have
> > to *think* about if they support Xen or not at development time. It
> > does this in a not-disruptive way to Xen / x86_64 but most
> > *importantly* it does not extend pvops! This should avoid issues in
> > cases of developer / maintainer bandwidth, should some new features be
> > pushed onto Linux for x86_64 but a respective Xen solution is not
> > addressed, and that was not caught early in patch review, such as with
> > Kasan.
> >
> > [0] 
> > https://lkml.kernel.org/r/1450217797-19295-1-git-send-email-mcg...@do-not-panic.com
> >
> > Two things I'd like to request a bit of help with and review / 
> > consideration:
> >
> > 1) I'd like some advice on a curious problem I've stumbled on. I'd
> > like to access hardware_subarch super early, and in my review with at
> > least two x86 folks this *should* work:
> >
> > diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
> > index c913b7eb5056..9168842821c8 100644
> > --- a/arch/x86/kernel/head64.c
> > +++ b/arch/x86/kernel/head64.c
> > @@ -141,6 +141,7 @@ static void __init copy_bootdata(char *real_mode_data)
> >
> >  asmlinkage __visible void __init x86_64_start_kernel(char * real_mode_data)
> >  {
> > + struct boot_params *params = (struct boot_params *)__va(real_mode_data);
> >   int i;
> 
> This is a mess :-p

Agreed. Doing what I can without extending pvops though ;)

> If you want to access real_mode_data before load_idt, you'll need to do:
> 
> for (i = 0; i < sizeof(boot_params); i += 4096)
> early_make_pgtable((unsigned long)params + i);

Thanks I'll give this a shot.

> Of course, it's entirely possible that that will blow up if you try to
> do it on Xen.

I'll check, if its safe and if the subarch strategy is desirable to help with
unifying init, then great. Otherwise we'd need to figure this out.

> I think this would all be easier to understand if you try to separate
> out the ideas of linker tables from the idea of rearranging early
> init.

Oh absolutely. The goal to unify init *or* to access subarch earlier provides
slightly different gains and possiblities. This is why I am addressing this
separately. Its important to highlight the prospects though given I think a few
folks may not have realized what might be possible here...

>  AFAICT the linker table thing is just an implementation detail.

Indeed, but just as a linker table is one thing, the *use* of the linker table
for x86 early init is another. Its a good example how how to use the linker
tables though.

The things I make mention of here are just possible *enhancements* of that work
provided the subarch can be read earlier.  Another possibility which I also had
not mentioned is the ability to also free annotated code on x86 init which we
*know* for sure we don't need, much as __init code after we boot, only this
could be done later at run time. That's also best technically considered later
but perhaps worth mentioning now as a future possibility.

Although the linker table series does not address unifying init, in this thread
we are talking about the prospect of being able to do that in the future. Its
best to consider this early than late.

> If I understand right, you're trying to unify the Xen and native
> startup as much as possible. 

That ultimately is a possibility. The original patches don't do that though.
They just pave the way with linker tables as baby steps.

Without access to the subarch so early unifying init is not possible with a
linker table solution though. As the series was posted though its late use
(after load_idt()) still holds promise to help annotate subarch support, which
in turn also helps provide dependency maps. This should help with a proactive
solution to ensure x86 developers think about x86 early requirements. Only
that gain would be 

Re: [Xen-devel] Unifying x86_64 / Xen init paths and reading hardware_subarch early

[H. Peter Anvin]

On January 15, 2016 4:43:04 PM PST, "Luis R. Rodriguez"  wrote:
>On Fri, Jan 15, 2016 at 03:47:25PM -0800, Andy Lutomirski wrote:
>> On Fri, Jan 15, 2016 at 2:08 PM, Luis R. Rodriguez 
>wrote:
>> > I will be respinning the generic Linux linker table solution [0]
>soon
>> > based on hpa's feedback again now that I'm back from vacation. As I
>do
>> > that though I wanted to highlight a feature I'm throwing into the
>> > linker table solution which I am not sure many have paid close
>> > attention to but I think is important to Xen. I'm making use of the
>> > zero page hardware_subarch to enable us to detect if we're a
>specific
>> > hypervisor solution *as early as is possible*. This has a few
>> > implications, short term it is designed to provides a proactive
>> > technical solution to bugs such as the cr4 shadow crash (see
>> > 5054daa285beaf706f051fbd395dc36c9f0f907f) and ensure that *new* x86
>> > features get a proper Xen implementation proactively *or* at the
>very
>> > least get annotated as unsupported properly, instead of having them
>> > crash and later finding out. A valid example here is Kasan, which
>to
>> > this day lacks proper Xen support. In the future, if the generic
>> > linker table solution gets merged, it would mean developers would
>have
>> > to *think* about if they support Xen or not at development time. It
>> > does this in a not-disruptive way to Xen / x86_64 but most
>> > *importantly* it does not extend pvops! This should avoid issues in
>> > cases of developer / maintainer bandwidth, should some new features
>be
>> > pushed onto Linux for x86_64 but a respective Xen solution is not
>> > addressed, and that was not caught early in patch review, such as
>with
>> > Kasan.
>> >
>> > [0]
>https://lkml.kernel.org/r/1450217797-19295-1-git-send-email-mcg...@do-not-panic.com
>> >
>> > Two things I'd like to request a bit of help with and review /
>consideration:
>> >
>> > 1) I'd like some advice on a curious problem I've stumbled on. I'd
>> > like to access hardware_subarch super early, and in my review with
>at
>> > least two x86 folks this *should* work:
>> >
>> > diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
>> > index c913b7eb5056..9168842821c8 100644
>> > --- a/arch/x86/kernel/head64.c
>> > +++ b/arch/x86/kernel/head64.c
>> > @@ -141,6 +141,7 @@ static void __init copy_bootdata(char
>*real_mode_data)
>> >
>> >  asmlinkage __visible void __init x86_64_start_kernel(char *
>real_mode_data)
>> >  {
>> > + struct boot_params *params = (struct boot_params
>*)__va(real_mode_data);
>> >   int i;
>> 
>> This is a mess :-p
>
>Agreed. Doing what I can without extending pvops though ;)
>
>> If you want to access real_mode_data before load_idt, you'll need to
>do:
>> 
>> for (i = 0; i < sizeof(boot_params); i += 4096)
>> early_make_pgtable((unsigned long)params + i);
>
>Thanks I'll give this a shot.
>
>> Of course, it's entirely possible that that will blow up if you try
>to
>> do it on Xen.
>
>I'll check, if its safe and if the subarch strategy is desirable to
>help with
>unifying init, then great. Otherwise we'd need to figure this out.
>
>> I think this would all be easier to understand if you try to separate
>> out the ideas of linker tables from the idea of rearranging early
>> init.
>
>Oh absolutely. The goal to unify init *or* to access subarch earlier
>provides
>slightly different gains and possiblities. This is why I am addressing
>this
>separately. Its important to highlight the prospects though given I
>think a few
>folks may not have realized what might be possible here...
>
>>  AFAICT the linker table thing is just an implementation detail.
>
>Indeed, but just as a linker table is one thing, the *use* of the
>linker table
>for x86 early init is another. Its a good example how how to use the
>linker
>tables though.
>
>The things I make mention of here are just possible *enhancements* of
>that work
>provided the subarch can be read earlier.  Another possibility which I
>also had
>not mentioned is the ability to also free annotated code on x86 init
>which we
>*know* for sure we don't need, much as __init code after we boot, only
>this
>could be done later at run time. That's also best technically
>considered later
>but perhaps worth mentioning now as a future possibility.
>
>Although the linker table series does not address unifying init, in
>this thread
>we are talking about the prospect of being able to do that in the
>future. Its
>best to consider this early than late.
>
>> If I understand right, you're trying to unify the Xen and native
>> startup as much as possible. 
>
>That ultimately is a possibility. The original patches don't do that
>though.
>They just pave the way with linker tables as baby steps.
>
>Without access to the subarch so early unifying init is not possible
>with a
>linker table solution though. As the series was posted though its late
>use
>(after load_idt()) still holds promise to help annotate 

Re: [Xen-devel] Unifying x86_64 / Xen init paths and reading hardware_subarch early

[Andy Lutomirski]

On Fri, Jan 15, 2016 at 2:08 PM, Luis R. Rodriguez  wrote:
> I will be respinning the generic Linux linker table solution [0] soon
> based on hpa's feedback again now that I'm back from vacation. As I do
> that though I wanted to highlight a feature I'm throwing into the
> linker table solution which I am not sure many have paid close
> attention to but I think is important to Xen. I'm making use of the
> zero page hardware_subarch to enable us to detect if we're a specific
> hypervisor solution *as early as is possible*. This has a few
> implications, short term it is designed to provides a proactive
> technical solution to bugs such as the cr4 shadow crash (see
> 5054daa285beaf706f051fbd395dc36c9f0f907f) and ensure that *new* x86
> features get a proper Xen implementation proactively *or* at the very
> least get annotated as unsupported properly, instead of having them
> crash and later finding out. A valid example here is Kasan, which to
> this day lacks proper Xen support. In the future, if the generic
> linker table solution gets merged, it would mean developers would have
> to *think* about if they support Xen or not at development time. It
> does this in a not-disruptive way to Xen / x86_64 but most
> *importantly* it does not extend pvops! This should avoid issues in
> cases of developer / maintainer bandwidth, should some new features be
> pushed onto Linux for x86_64 but a respective Xen solution is not
> addressed, and that was not caught early in patch review, such as with
> Kasan.
>
> [0] 
> https://lkml.kernel.org/r/1450217797-19295-1-git-send-email-mcg...@do-not-panic.com
>
> Two things I'd like to request a bit of help with and review / consideration:
>
> 1) I'd like some advice on a curious problem I've stumbled on. I'd
> like to access hardware_subarch super early, and in my review with at
> least two x86 folks this *should* work:
>
> diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
> index c913b7eb5056..9168842821c8 100644
> --- a/arch/x86/kernel/head64.c
> +++ b/arch/x86/kernel/head64.c
> @@ -141,6 +141,7 @@ static void __init copy_bootdata(char *real_mode_data)
>
>  asmlinkage __visible void __init x86_64_start_kernel(char * real_mode_data)
>  {
> + struct boot_params *params = (struct boot_params *)__va(real_mode_data);
>   int i;

This is a mess :-p

If you want to access real_mode_data before load_idt, you'll need to do:

for (i = 0; i < sizeof(boot_params); i += 4096)
early_make_pgtable((unsigned long)params + i);

Of course, it's entirely possible that that will blow up if you try to
do it on Xen.

I think this would all be easier to understand if you try to separate
out the ideas of linker tables from the idea of rearranging early
init.  AFAICT the linker table thing is just an implementation detail.

If I understand right, you're trying to unify the Xen and native
startup as much as possible.  Why not add little shims, though?
Create a new start_kernel_common(int subarch, ...) where subarch
indicates native vs Xen and have its callers tell it which mode it's
in?

--Andy

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Re: [Xen-devel] Unifying x86_64 / Xen init paths and reading hardware_subarch early

[Luis R. Rodriguez]

On Fri, Jan 15, 2016 at 4:43 PM, Luis R. Rodriguez  wrote:
>> for (i = 0; i < sizeof(boot_params); i += 4096)
>> early_make_pgtable((unsigned long)params + i);
>
>  I'll give this a shot.

Thanks again for this! It seems to let this boot now! But it does not
seem to provided the right value. If I use the qemu debug patch as I
listed before to set this to 5 for kvm, and boot it doesn't come up.
This can be tested with the qemu debug patch + this debug kernel patch
which prints it out and resets it from what it finds early.

If you comment out the boot_params.hdr.hardware_subarch =
my_hardware_subarch; assignment we get the right value from the
copy_bootdata() work. I use my_hardware_subarch just as a quick hack
to test and cache the value early code gets but that I can't print
early on.

diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
index c913b7eb5056..6fc92553f272 100644
--- a/arch/x86/kernel/head64.c
+++ b/arch/x86/kernel/head64.c
@@ -139,9 +139,12 @@ static void __init copy_bootdata(char *real_mode_data)
  }
 }

+__u32 my_hardware_subarch;
+
 asmlinkage __visible void __init x86_64_start_kernel(char * real_mode_data)
 {
  int i;
+ struct boot_params *params = (struct boot_params *)__va(real_mode_data);

  /*
  * Build-time sanity checks on the kernel image and module
@@ -157,6 +160,13 @@ asmlinkage __visible void __init
x86_64_start_kernel(char * real_mode_data)
  (__START_KERNEL & PGDIR_MASK)));
  BUILD_BUG_ON(__fix_to_virt(__end_of_fixed_addresses) <= MODULES_END);

+ /* Make the zero page accessible as early as possible */
+ for (i = 0; i < sizeof(boot_params); i += 4096)
+ early_make_pgtable((unsigned long)params + i);
+
+ boot_params.hdr.hardware_subarch = params->hdr.hardware_subarch;
+ my_hardware_subarch = params->hdr.hardware_subarch;
+
  cr4_init_shadow();

  /* Kill off the identity-map trampoline */
@@ -173,6 +183,7 @@ asmlinkage __visible void __init
x86_64_start_kernel(char * real_mode_data)
  load_idt((const struct desc_ptr *)_descr);

  copy_bootdata(__va(real_mode_data));
+ boot_params.hdr.hardware_subarch = my_hardware_subarch;

  /*
  * Load microcode early on BSP.
diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
index d3d80e6d42a2..c2f85f8ab52b 100644
--- a/arch/x86/kernel/setup.c
+++ b/arch/x86/kernel/setup.c
@@ -851,6 +851,8 @@ void __init setup_arch(char **cmdline_p)
  (unsigned long)__bss_stop - (unsigned long)_text);

  early_reserve_initrd();
+ pr_info("boot_params.hdr.hardware_subarch: 0x%04x\n",
+ boot_params.hdr.hardware_subarch);

  /*
  * At this point everything still needed from the boot loader


  Luis

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