Re: [Xen-devel] [PATCH 1/3] x86/xen: Avoid fast syscall path for Xen PV guests
On Wed, Nov 18, 2015 at 12:21:56PM -0800, Andy Lutomirski wrote: > Could we make this a little less subtle: > > ALTERNATIVE "testl %eax, %eax; lz .Lsyscall_32_done", "jmp > .Lsyscasll_32_done", X86_FEATURE_XENPV > > Borislav, what do you think? I don't mind either. I would've said your version doesn't touch %eax so the result in there might be useful for callers but all paths do overwrite it, AFAICT. -- Regards/Gruss, Boris. SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg) -- ___ Xen-devel mailing list Xen-devel@lists.xen.org http://lists.xen.org/xen-devel
Re: [Xen-devel] [PATCH 1/3] x86/xen: Avoid fast syscall path for Xen PV guests
On Wed, Nov 18, 2015 at 3:21 PM, Andy Lutomirski wrote: > On Wed, Nov 18, 2015 at 12:06 PM, Boris Ostrovsky > wrote: >> After 32-bit syscall rewrite, and specifically after commit 5f310f739b4c >> ("x86/entry/32: Re-implement SYSENTER using the new C path"), the stack >> frame that is passed to xen_sysexit is no longer a "standard" one (i.e. >> it's not pt_regs). >> >> Since we end up calling xen_iret from xen_sysexit we don't need to fix >> up the stack and instead follow entry_SYSENTER_32's IRET path directly >> to xen_iret. >> >> We can do the same thing for compat mode even though stack does not need >> to be fixed. This will allow us to drop usergs_sysret32 paravirt op (in >> the subsequent patch) > > Looks generally quite nice. Minor comments below: > >> --- a/arch/x86/entry/entry_32.S >> +++ b/arch/x86/entry/entry_32.S >> @@ -308,7 +308,8 @@ sysenter_past_esp: >> >> movl%esp, %eax >> calldo_fast_syscall_32 >> - testl %eax, %eax >> + /* XEN PV guests always use IRET path */ >> + ALTERNATIVE "testl %eax, %eax", "xor %eax, %eax", X86_FEATURE_XENPV >> jz .Lsyscall_32_done > > Could we make this a little less subtle: > > ALTERNATIVE "testl %eax, %eax; lz .Lsyscall_32_done", "jmp > .Lsyscasll_32_done", X86_FEATURE_XENPV > > Borislav, what do you think? > > Ditto for the others. Can you just add !xen_pv_domain() to the opportunistic SYSRET check instead? Bury the alternatives in that macro, ie. static_cpu_has(X86_FEATURE_XENPV). That would likely benefit other code as well. -- Brian Gerst ___ Xen-devel mailing list Xen-devel@lists.xen.org http://lists.xen.org/xen-devel
Re: [Xen-devel] [PATCH 1/3] x86/xen: Avoid fast syscall path for Xen PV guests
On 11/18/2015 03:58 PM, Andy Lutomirski wrote: On Wed, Nov 18, 2015 at 12:50 PM, Brian Gerst wrote: Can you just add !xen_pv_domain() to the opportunistic SYSRET check instead? Bury the alternatives in that macro, ie. static_cpu_has(X86_FEATURE_XENPV). That would likely benefit other code as well. We could, but that won't help the 64-bit case where we want to keep the full asm path. I don't think I understand what you mean here. Which full asm path? -boris ___ Xen-devel mailing list Xen-devel@lists.xen.org http://lists.xen.org/xen-devel
Re: [Xen-devel] [PATCH 1/3] x86/xen: Avoid fast syscall path for Xen PV guests
On Wed, Nov 18, 2015 at 12:50 PM, Brian Gerst wrote: > On Wed, Nov 18, 2015 at 3:21 PM, Andy Lutomirski wrote: >> On Wed, Nov 18, 2015 at 12:06 PM, Boris Ostrovsky >> wrote: >>> After 32-bit syscall rewrite, and specifically after commit 5f310f739b4c >>> ("x86/entry/32: Re-implement SYSENTER using the new C path"), the stack >>> frame that is passed to xen_sysexit is no longer a "standard" one (i.e. >>> it's not pt_regs). >>> >>> Since we end up calling xen_iret from xen_sysexit we don't need to fix >>> up the stack and instead follow entry_SYSENTER_32's IRET path directly >>> to xen_iret. >>> >>> We can do the same thing for compat mode even though stack does not need >>> to be fixed. This will allow us to drop usergs_sysret32 paravirt op (in >>> the subsequent patch) >> >> Looks generally quite nice. Minor comments below: >> >>> --- a/arch/x86/entry/entry_32.S >>> +++ b/arch/x86/entry/entry_32.S >>> @@ -308,7 +308,8 @@ sysenter_past_esp: >>> >>> movl%esp, %eax >>> calldo_fast_syscall_32 >>> - testl %eax, %eax >>> + /* XEN PV guests always use IRET path */ >>> + ALTERNATIVE "testl %eax, %eax", "xor %eax, %eax", X86_FEATURE_XENPV >>> jz .Lsyscall_32_done >> >> Could we make this a little less subtle: >> >> ALTERNATIVE "testl %eax, %eax; lz .Lsyscall_32_done", "jmp >> .Lsyscasll_32_done", X86_FEATURE_XENPV >> >> Borislav, what do you think? >> >> Ditto for the others. > > Can you just add !xen_pv_domain() to the opportunistic SYSRET check > instead? Bury the alternatives in that macro, ie. > static_cpu_has(X86_FEATURE_XENPV). That would likely benefit other > code as well. We could, but that won't help the 64-bit case where we want to keep the full asm path. Also, Xen is capable of the equivalent of sysret32 in the compat case. We might want to enable something like that, and using the existing opportunistic sysret check may make sense, in which case we wouldn't want to disable it. --Andy ___ Xen-devel mailing list Xen-devel@lists.xen.org http://lists.xen.org/xen-devel
Re: [Xen-devel] [PATCH 1/3] x86/xen: Avoid fast syscall path for Xen PV guests
On Wed, Nov 18, 2015 at 12:21:56PM -0800, Andy Lutomirski wrote: > > diff --git a/arch/x86/include/asm/cpufeature.h > > b/arch/x86/include/asm/cpufeature.h > > index e4f8010..0e4fe5b 100644 > > --- a/arch/x86/include/asm/cpufeature.h > > +++ b/arch/x86/include/asm/cpufeature.h > > @@ -216,6 +216,7 @@ > > #define X86_FEATURE_PAUSEFILTER ( 8*32+13) /* AMD filtered pause intercept > > */ > > #define X86_FEATURE_PFTHRESHOLD ( 8*32+14) /* AMD pause filter threshold */ > > #define X86_FEATURE_VMMCALL ( 8*32+15) /* Prefer vmmcall to vmcall */ > > +#define X86_FEATURE_XENPV ( 8*32+16) /* Xen paravirtual guest */ > > > > This bit is highly magical and I think we need Borislav's ack. Yeah, that should be #define X86_FEATURE_XENPV ( 8*32+16) /* "" Xen paravirtual guest */ ^^ note the empty "". This prevents it from appearing in /proc/cpuinfo. -- Regards/Gruss, Boris. SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg) -- ___ Xen-devel mailing list Xen-devel@lists.xen.org http://lists.xen.org/xen-devel
Re: [Xen-devel] [PATCH 1/3] x86/xen: Avoid fast syscall path for Xen PV guests
On Wed, Nov 18, 2015 at 12:06 PM, Boris Ostrovsky wrote: > After 32-bit syscall rewrite, and specifically after commit 5f310f739b4c > ("x86/entry/32: Re-implement SYSENTER using the new C path"), the stack > frame that is passed to xen_sysexit is no longer a "standard" one (i.e. > it's not pt_regs). > > Since we end up calling xen_iret from xen_sysexit we don't need to fix > up the stack and instead follow entry_SYSENTER_32's IRET path directly > to xen_iret. > > We can do the same thing for compat mode even though stack does not need > to be fixed. This will allow us to drop usergs_sysret32 paravirt op (in > the subsequent patch) Looks generally quite nice. Minor comments below: > --- a/arch/x86/entry/entry_32.S > +++ b/arch/x86/entry/entry_32.S > @@ -308,7 +308,8 @@ sysenter_past_esp: > > movl%esp, %eax > calldo_fast_syscall_32 > - testl %eax, %eax > + /* XEN PV guests always use IRET path */ > + ALTERNATIVE "testl %eax, %eax", "xor %eax, %eax", X86_FEATURE_XENPV > jz .Lsyscall_32_done Could we make this a little less subtle: ALTERNATIVE "testl %eax, %eax; lz .Lsyscall_32_done", "jmp .Lsyscasll_32_done", X86_FEATURE_XENPV Borislav, what do you think? Ditto for the others. > diff --git a/arch/x86/include/asm/cpufeature.h > b/arch/x86/include/asm/cpufeature.h > index e4f8010..0e4fe5b 100644 > --- a/arch/x86/include/asm/cpufeature.h > +++ b/arch/x86/include/asm/cpufeature.h > @@ -216,6 +216,7 @@ > #define X86_FEATURE_PAUSEFILTER ( 8*32+13) /* AMD filtered pause intercept */ > #define X86_FEATURE_PFTHRESHOLD ( 8*32+14) /* AMD pause filter threshold */ > #define X86_FEATURE_VMMCALL ( 8*32+15) /* Prefer vmmcall to vmcall */ > +#define X86_FEATURE_XENPV ( 8*32+16) /* Xen paravirtual guest */ > This bit is highly magical and I think we need Borislav's ack. --Andy ___ Xen-devel mailing list Xen-devel@lists.xen.org http://lists.xen.org/xen-devel
[Xen-devel] [PATCH 1/3] x86/xen: Avoid fast syscall path for Xen PV guests
After 32-bit syscall rewrite, and specifically after commit 5f310f739b4c ("x86/entry/32: Re-implement SYSENTER using the new C path"), the stack frame that is passed to xen_sysexit is no longer a "standard" one (i.e. it's not pt_regs). Since we end up calling xen_iret from xen_sysexit we don't need to fix up the stack and instead follow entry_SYSENTER_32's IRET path directly to xen_iret. We can do the same thing for compat mode even though stack does not need to be fixed. This will allow us to drop usergs_sysret32 paravirt op (in the subsequent patch) Signed-off-by: Boris Ostrovsky Suggested-by: Andy Lutomirski --- arch/x86/entry/entry_32.S | 3 ++- arch/x86/entry/entry_64_compat.S | 6 -- arch/x86/include/asm/cpufeature.h | 1 + arch/x86/xen/enlighten.c | 4 +++- 4 files changed, 10 insertions(+), 4 deletions(-) diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S index 3eb572e..901f186 100644 --- a/arch/x86/entry/entry_32.S +++ b/arch/x86/entry/entry_32.S @@ -308,7 +308,8 @@ sysenter_past_esp: movl%esp, %eax calldo_fast_syscall_32 - testl %eax, %eax + /* XEN PV guests always use IRET path */ + ALTERNATIVE "testl %eax, %eax", "xor %eax, %eax", X86_FEATURE_XENPV jz .Lsyscall_32_done /* Opportunistic SYSEXIT */ diff --git a/arch/x86/entry/entry_64_compat.S b/arch/x86/entry/entry_64_compat.S index c320183..98893d9 100644 --- a/arch/x86/entry/entry_64_compat.S +++ b/arch/x86/entry/entry_64_compat.S @@ -121,7 +121,8 @@ sysenter_flags_fixed: movq%rsp, %rdi calldo_fast_syscall_32 - testl %eax, %eax + /* XEN PV guests always use IRET path */ + ALTERNATIVE "testl %eax, %eax", "xor %eax, %eax", X86_FEATURE_XENPV jz .Lsyscall_32_done jmp sysret32_from_system_call @@ -200,7 +201,8 @@ ENTRY(entry_SYSCALL_compat) movq%rsp, %rdi calldo_fast_syscall_32 - testl %eax, %eax + /* XEN PV guests always use IRET path */ + ALTERNATIVE "testl %eax, %eax", "xor %eax, %eax", X86_FEATURE_XENPV jz .Lsyscall_32_done /* Opportunistic SYSRET */ diff --git a/arch/x86/include/asm/cpufeature.h b/arch/x86/include/asm/cpufeature.h index e4f8010..0e4fe5b 100644 --- a/arch/x86/include/asm/cpufeature.h +++ b/arch/x86/include/asm/cpufeature.h @@ -216,6 +216,7 @@ #define X86_FEATURE_PAUSEFILTER ( 8*32+13) /* AMD filtered pause intercept */ #define X86_FEATURE_PFTHRESHOLD ( 8*32+14) /* AMD pause filter threshold */ #define X86_FEATURE_VMMCALL ( 8*32+15) /* Prefer vmmcall to vmcall */ +#define X86_FEATURE_XENPV ( 8*32+16) /* Xen paravirtual guest */ /* Intel-defined CPU features, CPUID level 0x0007:0 (ebx), word 9 */ diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c index 5774800..d315151 100644 --- a/arch/x86/xen/enlighten.c +++ b/arch/x86/xen/enlighten.c @@ -1886,8 +1886,10 @@ EXPORT_SYMBOL_GPL(xen_hvm_need_lapic); static void xen_set_cpu_features(struct cpuinfo_x86 *c) { - if (xen_pv_domain()) + if (xen_pv_domain()) { clear_cpu_bug(c, X86_BUG_SYSRET_SS_ATTRS); + set_cpu_cap(c, X86_FEATURE_XENPV); + } } const struct hypervisor_x86 x86_hyper_xen = { -- 1.8.1.4 ___ Xen-devel mailing list Xen-devel@lists.xen.org http://lists.xen.org/xen-devel