Re: [edk2-devel] [PATCH v1 2/2] UefiCpuPkg/PiSmmCpuDxeSmm: Check BspIndex first before lock cmpxchg

["Ni, Ray" <ray.ni@intel.com>]

> 
> This patch makes me uncomfortable. I understand what it intends to do,
> and the intent is not wrong, but we're again treating "volatile UINT32"
> as atomic, and non-reorderable against the
> InterlockedCompareExchange32(). This kind of "optimization" is what
> people write cautionary tales about, later. It would be nice to see a
> semi-formal *proof* that this cannot backfire.

Laszlo,
Test-and-set is implemented in x86 through the cmpxchg instruction.

The patch follows https://en.wikipedia.org/wiki/Test_and_test-and-set
and adds a "test" before "test-and-set".

> 
> Either way: I'm not trying to block the patch. If Ray is happy with it,
> I don't object. OVMF implements PlatformSmmBspElection() in
> "OvmfPkg/Library/SmmCpuPlatformHookLibQemu/SmmCpuPlatformHookLi
> bQemu.c",
> always returning EFI_SUCCESS [*], so the code that is being modified
> here cannot be reached in OVMF.
> 
> [*] commit 43df61878d94 ("OvmfPkg: enable SMM Monarch Election in
> PiSmmCpuDxeSmm", 2020-03-04)
> 
> Laszlo
> 
> 
> 
> 
> 



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