Thank all for review.
> Volatile-qualifying the local variables does not seem useful for
> anything. It's fine -- actually: it's beneficial -- if the compiler
> optimizes accesses to those locals -- being on the stack -- as heavily
> as it can. In other words, those parts of the patch look like a small
> performance regression.
Obviously, local variables like LocalPleB just a temporary variable.
From optimization perspective, it may be replaced by function parameter PleB.
Qualifying local variable is for preventing it is optimized. In this case, code
like "LocalPleB.Bits.Present = Attribute->Bits.Present" may lead to unexpected
result, as it is not atomic.
It might lead to performance degradation, as it can't be optimized to register.
I think such code will not be called in large quantities and slight drop in
performance is acceptable.
> IN OUT volatile IA32_PAGE_NON_LEAF_ENTRY *Pnle
> and similar. Then the existent assignment expressions
> Pnle->Uint64 = LocalPnle.Uint64;
> don't have to be changed.
agree that such improvement would have the same effect, and the code would be a
little more graceful.
Will update this patch.
Thanks & Regards,
Zhou Jianfeng
-----Original Message-----
From: Pedro Falcato <pedro.falc...@gmail.com>
Sent: Thursday, February 22, 2024 5:44 AM
To: devel@edk2.groups.io; ler...@redhat.com
Cc: Zhou, Jianfeng <jianfeng.z...@intel.com>; Ni, Ray <ray...@intel.com>;
Kumar, Rahul R <rahul.r.ku...@intel.com>; Gerd Hoffmann <kra...@redhat.com>
Subject: Re: [edk2-devel] [PATCH] UefiCpuPkg: add volatile qualifier to page
table related variable
On Wed, Feb 21, 2024 at 8:36 PM Laszlo Ersek <ler...@redhat.com> wrote:
>
> On 2/21/24 02:25, Zhou Jianfeng wrote:
> > Add volatile qualifier to page table related variable to prevent
> > compiler from optimizing away the variables which may lead to
> > unexpected result.
> >
> > Signed-off-by: Zhou Jianfeng <jianfeng.z...@intel.com>
> > Cc: Ray Ni <ray...@intel.com>
> > Cc: Laszlo Ersek <ler...@redhat.com>
> > Cc: Rahul Kumar <rahul1.ku...@intel.com>
> > Cc: Gerd Hoffmann <kra...@redhat.com>
I'd appreciate getting CC'd on my own suggestion....
> > ---
> > UefiCpuPkg/Library/CpuPageTableLib/CpuPageTableMap.c | 12
> > ++++++------
> > 1 file changed, 6 insertions(+), 6 deletions(-)
>
> (1) subject should be something like:
>
> UefiCpuPkg/CpuPageTableLib: qualify page table accesses as volatile
>
> >
> > diff --git a/UefiCpuPkg/Library/CpuPageTableLib/CpuPageTableMap.c
> > b/UefiCpuPkg/Library/CpuPageTableLib/CpuPageTableMap.c
> > index 2ea40666cc..5cf6e8fea0 100644
> > --- a/UefiCpuPkg/Library/CpuPageTableLib/CpuPageTableMap.c
> > +++ b/UefiCpuPkg/Library/CpuPageTableLib/CpuPageTableMap.c
> > @@ -26,7 +26,7 @@ PageTableLibSetPte4K (
> > IN IA32_MAP_ATTRIBUTE *Mask
> > )
> > {
> > - IA32_PTE_4K LocalPte4K;
> > + volatile IA32_PTE_4K LocalPte4K;
> >
> > LocalPte4K.Uint64 = Pte4K->Uint64;
> > if (Mask->Bits.PageTableBaseAddressLow ||
> > Mask->Bits.PageTableBaseAddressHigh) { @@ -78,7 +78,7 @@
> > PageTableLibSetPte4K (
> > }
> >
> > if (Pte4K->Uint64 != LocalPte4K.Uint64) {
> > - Pte4K->Uint64 = LocalPte4K.Uint64;
> > + *(volatile UINT64 *)&(Pte4K->Uint64) = LocalPte4K.Uint64;
> > }
> > }
> >
> > @@ -100,7 +100,7 @@ PageTableLibSetPleB (
> > IN IA32_MAP_ATTRIBUTE *Mask
> > )
> > {
> > - IA32_PAGE_LEAF_ENTRY_BIG_PAGESIZE LocalPleB;
> > + volatile IA32_PAGE_LEAF_ENTRY_BIG_PAGESIZE LocalPleB;
> >
> > LocalPleB.Uint64 = PleB->Uint64;
> > if (Mask->Bits.PageTableBaseAddressLow ||
> > Mask->Bits.PageTableBaseAddressHigh) { @@ -154,7 +154,7 @@
> > PageTableLibSetPleB (
> > }
> >
> > if (PleB->Uint64 != LocalPleB.Uint64) {
> > - PleB->Uint64 = LocalPleB.Uint64;
> > + *(volatile UINT64 *)&(PleB->Uint64) = LocalPleB.Uint64;
> > }
> > }
> >
> > @@ -200,7 +200,7 @@ PageTableLibSetPnle (
> > IN IA32_MAP_ATTRIBUTE *Mask
> > )
> > {
> > - IA32_PAGE_NON_LEAF_ENTRY LocalPnle;
> > + volatile IA32_PAGE_NON_LEAF_ENTRY LocalPnle;
> >
> > LocalPnle.Uint64 = Pnle->Uint64;
> > if (Mask->Bits.Present) {
> > @@ -231,7 +231,7 @@ PageTableLibSetPnle (
> > LocalPnle.Bits.WriteThrough = 0;
> > LocalPnle.Bits.CacheDisabled = 0;
> > if (Pnle->Uint64 != LocalPnle.Uint64) {
> > - Pnle->Uint64 = LocalPnle.Uint64;
> > + *(volatile UINT64 *)&(Pnle->Uint64) = LocalPnle.Uint64;
> > }
> > }
>
> I agree with the idea (I think it's a necessary change, or put
> differently, an improvement, even though I may not be convinced that
> it is a *sufficient* improvement; but let's not rehash all that here
> again); however, I think the implementation is not the greatest.
>
> Volatile-qualifying the local variables does not seem useful for
> anything. It's fine -- actually: it's beneficial -- if the compiler
> optimizes accesses to those locals -- being on the stack -- as heavily
> as it can. In other words, those parts of the patch look like a small
> performance regression.
>
> (2) What we want to qualify as volatile here are the *targets* of the
> Pte4K, PleB and Pnle pointers. Your other patch ("UefiCpuPkg: Fix IN
> OUT parameters marked as IN") correctly marks those as "IN OUT", so in
> this patch, we should update them to:
>
> IN OUT volatile IA32_PAGE_NON_LEAF_ENTRY *Pnle
>
> and similar. Then the existent assignment expressions
>
> Pnle->Uint64 = LocalPnle.Uint64;
>
> don't have to be changed.
I echo these comments :)
>
> Note that call sites will not have to be updated either; see C99
> 6.3.2.3 Pointers, paragraph 2:
>
> For any qualifier q, a pointer to a non-q-qualified type may be
> converted to a pointer to the q-qualified version of the type; the
> values stored in the original and converted pointers shall compare
> equal.
Ugh, honestly converting to volatile implicitly is kind-of yucky, but I guess
it works; personally I'd rather have explicit conversion, but it's just a
matter of taste.
What I *really* prefer in these cases (when we're not dealing with
MMIO) is something like READ_ONCE and WRITE_ONCE, where the "volatility points"
are very well annotated, but oh well :)
--
Pedro
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