Re: [edk2-devel] [PATCH] UefiCpuPkg/CpuPageTableLib: qualify page table accesses as volatile

["Zhou, Jianfeng" <jianfeng.zhou@intel.com>]

> While it may well cause the compiler to generate less optimised code, there is absolutely no way that this volatile declaration on a local stack variable can possibly change the outcome of the code.
> There can never be any meaningful side-effects from reading or writing a stack variable.
> I would suggest dropping the volatile on LocalPte4K, since its *only* possible impact is to confuse a future reader of the code.

The change is for preventing compiler from optimizing.
As a temporary variable,  LocalPte4K may be replaced by function parameter Pte4K.
In this case, code like "LocalPte4K.Bits.Present = Attribute->Bits.Present" may lead to unexpected result, as it is not atomic. Assembly code look like:
   mov eax, [r8]
   and dword [rcx], 0xfffffffe  // this instruction clear the present bit and may leads to unexpected result.
   and eax, 0x1
   or [rcx], eax

Thanks & Regards,
Zhou Jianfeng

-----Original Message-----
From: Michael Brown <mcb30@ipxe.org> 
Sent: Friday, February 23, 2024 7:59 PM
To: devel@edk2.groups.io; Zhou, Jianfeng <jianfeng.zhou@intel.com>
Cc: Ni, Ray <ray.ni@intel.com>; Laszlo Ersek <lersek@redhat.com>; Kumar, Rahul R <rahul.r.kumar@intel.com>; Gerd Hoffmann <kraxel@redhat.com>; Pedro Falcato <pedro.falcato@gmail.com>; Zhang, Di <di.zhang@intel.com>; Tan, Dun <dun.tan@intel.com>
Subject: Re: [edk2-devel] [PATCH] UefiCpuPkg/CpuPageTableLib: qualify page table accesses as volatile

On 22/02/2024 08:41, Zhou Jianfeng wrote:
> --- a/UefiCpuPkg/Library/CpuPageTableLib/CpuPageTableMap.c
> +++ b/UefiCpuPkg/Library/CpuPageTableLib/CpuPageTableMap.c
> @@ -20,13 +20,13 @@
>   **/
>   VOID
>   PageTableLibSetPte4K (
> -  IN OUT IA32_PTE_4K         *Pte4K,
> -  IN UINT64                  Offset,
> -  IN IA32_MAP_ATTRIBUTE      *Attribute,
> -  IN IA32_MAP_ATTRIBUTE      *Mask
> +  IN OUT volatile IA32_PTE_4K  *Pte4K,
> +  IN UINT64                    Offset,
> +  IN IA32_MAP_ATTRIBUTE        *Attribute,
> +  IN IA32_MAP_ATTRIBUTE        *Mask
>     )
>   {
> -  IA32_PTE_4K  LocalPte4K;
> +  volatile IA32_PTE_4K  LocalPte4K;

While it may well cause the compiler to generate less optimised code, there is absolutely no way that this volatile declaration on a local stack variable can possibly change the outcome of the code.

There can never be any meaningful side-effects from reading or writing a stack variable.

I would suggest dropping the volatile on LocalPte4K, since its *only* possible impact is to confuse a future reader of the code.

> -  IA32_PAGE_LEAF_ENTRY_BIG_PAGESIZE  LocalPleB;
> +  volatile IA32_PAGE_LEAF_ENTRY_BIG_PAGESIZE  LocalPleB;

Same comment.

> -  IA32_PAGE_NON_LEAF_ENTRY  LocalPnle;
> +  volatile IA32_PAGE_NON_LEAF_ENTRY  LocalPnle;

Same comment.

Thanks,

Michael



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