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

["Michael Brown" <mcb30@ipxe.org>]

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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