Re: [edk2-devel] [PATCH 1/3] UefiCpuPkg: Reduce and optimize access to attribute

["Laszlo Ersek" <lersek@redhat.com>]

On 2/5/24 15:03, duntan wrote:
> From: Zhou Jianfeng <jianfeng.zhou@intel.com>
> 
> This commit is to reduce and optimize access to
> attribute in CpuPageTableLib.
> 
> Unreasonable writing to attribute of page table may
> leads to expection.
> The assembly code for C code Pnle->Bits.Present =
> Attribute->Bits.Present looks like:
>    and dword [rcx], 0xfffffffe
>    and eax, 0x1
>    or [rcx], eax
> In case Pnle->Bits.Present and Attribute->Bits.Present
> is 1, Pnle->Bits.Present will be set to 0 for short
> time(2 instructions) which is unexpected. If some other
> core is accessing the page, it may leads to expection.
> This change reduce and optimize access to attribute of
> page table, attribute of page table is set only when it
> need to be changed.

This patch does nothing to eliminate the actual race condition, it only
shrinks the window of potential corruption.

The PTEs continue to be overwritten without any kind of synchronization
with the other processors.

Feel free to merge this with Ray's R-b.

Laszlo

> 
> Signed-off-by: Zhou Jianfeng <jianfeng.zhou@intel.com>
> Cc: Ray Ni <ray.ni@intel.com>
> Cc: Laszlo Ersek <lersek@redhat.com>
> Cc: Rahul Kumar <rahul1.kumar@intel.com>
> Cc: Gerd Hoffmann <kraxel@redhat.com>
> ---
>  UefiCpuPkg/Library/CpuPageTableLib/CpuPageTableMap.c | 86 +++++++++++++++++++++++++++++++++++++++++++++++++++++---------------------------------
>  1 file changed, 53 insertions(+), 33 deletions(-)
> 
> diff --git a/UefiCpuPkg/Library/CpuPageTableLib/CpuPageTableMap.c b/UefiCpuPkg/Library/CpuPageTableLib/CpuPageTableMap.c
> index 36b2c4e6a3..ae4caf8dfe 100644
> --- a/UefiCpuPkg/Library/CpuPageTableLib/CpuPageTableMap.c
> +++ b/UefiCpuPkg/Library/CpuPageTableLib/CpuPageTableMap.c
> @@ -26,52 +26,59 @@ PageTableLibSetPte4K (
>    IN IA32_MAP_ATTRIBUTE  *Mask
>    )
>  {
> +  IA32_PTE_4K  LocalPte4K;
> +
> +  LocalPte4K.Uint64 = Pte4K->Uint64;
>    if (Mask->Bits.PageTableBaseAddressLow || Mask->Bits.PageTableBaseAddressHigh) {
> -    Pte4K->Uint64 = (IA32_MAP_ATTRIBUTE_PAGE_TABLE_BASE_ADDRESS (Attribute) + Offset) | (Pte4K->Uint64 & ~IA32_PE_BASE_ADDRESS_MASK_40);
> +    LocalPte4K.Uint64 = (IA32_MAP_ATTRIBUTE_PAGE_TABLE_BASE_ADDRESS (Attribute) + Offset) | (Pte4K->Uint64 & ~IA32_PE_BASE_ADDRESS_MASK_40);
>    }
>  
>    if (Mask->Bits.Present) {
> -    Pte4K->Bits.Present = Attribute->Bits.Present;
> +    LocalPte4K.Bits.Present = Attribute->Bits.Present;
>    }
>  
>    if (Mask->Bits.ReadWrite) {
> -    Pte4K->Bits.ReadWrite = Attribute->Bits.ReadWrite;
> +    LocalPte4K.Bits.ReadWrite = Attribute->Bits.ReadWrite;
>    }
>  
>    if (Mask->Bits.UserSupervisor) {
> -    Pte4K->Bits.UserSupervisor = Attribute->Bits.UserSupervisor;
> +    LocalPte4K.Bits.UserSupervisor = Attribute->Bits.UserSupervisor;
>    }
>  
>    if (Mask->Bits.WriteThrough) {
> -    Pte4K->Bits.WriteThrough = Attribute->Bits.WriteThrough;
> +    LocalPte4K.Bits.WriteThrough = Attribute->Bits.WriteThrough;
>    }
>  
>    if (Mask->Bits.CacheDisabled) {
> -    Pte4K->Bits.CacheDisabled = Attribute->Bits.CacheDisabled;
> +    LocalPte4K.Bits.CacheDisabled = Attribute->Bits.CacheDisabled;
>    }
>  
>    if (Mask->Bits.Accessed) {
> -    Pte4K->Bits.Accessed = Attribute->Bits.Accessed;
> +    LocalPte4K.Bits.Accessed = Attribute->Bits.Accessed;
>    }
>  
>    if (Mask->Bits.Dirty) {
> -    Pte4K->Bits.Dirty = Attribute->Bits.Dirty;
> +    LocalPte4K.Bits.Dirty = Attribute->Bits.Dirty;
>    }
>  
>    if (Mask->Bits.Pat) {
> -    Pte4K->Bits.Pat = Attribute->Bits.Pat;
> +    LocalPte4K.Bits.Pat = Attribute->Bits.Pat;
>    }
>  
>    if (Mask->Bits.Global) {
> -    Pte4K->Bits.Global = Attribute->Bits.Global;
> +    LocalPte4K.Bits.Global = Attribute->Bits.Global;
>    }
>  
>    if (Mask->Bits.ProtectionKey) {
> -    Pte4K->Bits.ProtectionKey = Attribute->Bits.ProtectionKey;
> +    LocalPte4K.Bits.ProtectionKey = Attribute->Bits.ProtectionKey;
>    }
>  
>    if (Mask->Bits.Nx) {
> -    Pte4K->Bits.Nx = Attribute->Bits.Nx;
> +    LocalPte4K.Bits.Nx = Attribute->Bits.Nx;
> +  }
> +
> +  if (Pte4K->Uint64 != LocalPte4K.Uint64) {
> +    Pte4K->Uint64 = LocalPte4K.Uint64;
>    }
>  }
>  
> @@ -93,54 +100,61 @@ PageTableLibSetPleB (
>    IN IA32_MAP_ATTRIBUTE                 *Mask
>    )
>  {
> +  IA32_PAGE_LEAF_ENTRY_BIG_PAGESIZE  LocalPleB;
> +
> +  LocalPleB.Uint64 = PleB->Uint64;
>    if (Mask->Bits.PageTableBaseAddressLow || Mask->Bits.PageTableBaseAddressHigh) {
> -    PleB->Uint64 = (IA32_MAP_ATTRIBUTE_PAGE_TABLE_BASE_ADDRESS (Attribute) + Offset) | (PleB->Uint64 & ~IA32_PE_BASE_ADDRESS_MASK_39);
> +    LocalPleB.Uint64 = (IA32_MAP_ATTRIBUTE_PAGE_TABLE_BASE_ADDRESS (Attribute) + Offset) | (PleB->Uint64 & ~IA32_PE_BASE_ADDRESS_MASK_39);
>    }
>  
> -  PleB->Bits.MustBeOne = 1;
> +  LocalPleB.Bits.MustBeOne = 1;
>  
>    if (Mask->Bits.Present) {
> -    PleB->Bits.Present = Attribute->Bits.Present;
> +    LocalPleB.Bits.Present = Attribute->Bits.Present;
>    }
>  
>    if (Mask->Bits.ReadWrite) {
> -    PleB->Bits.ReadWrite = Attribute->Bits.ReadWrite;
> +    LocalPleB.Bits.ReadWrite = Attribute->Bits.ReadWrite;
>    }
>  
>    if (Mask->Bits.UserSupervisor) {
> -    PleB->Bits.UserSupervisor = Attribute->Bits.UserSupervisor;
> +    LocalPleB.Bits.UserSupervisor = Attribute->Bits.UserSupervisor;
>    }
>  
>    if (Mask->Bits.WriteThrough) {
> -    PleB->Bits.WriteThrough = Attribute->Bits.WriteThrough;
> +    LocalPleB.Bits.WriteThrough = Attribute->Bits.WriteThrough;
>    }
>  
>    if (Mask->Bits.CacheDisabled) {
> -    PleB->Bits.CacheDisabled = Attribute->Bits.CacheDisabled;
> +    LocalPleB.Bits.CacheDisabled = Attribute->Bits.CacheDisabled;
>    }
>  
>    if (Mask->Bits.Accessed) {
> -    PleB->Bits.Accessed = Attribute->Bits.Accessed;
> +    LocalPleB.Bits.Accessed = Attribute->Bits.Accessed;
>    }
>  
>    if (Mask->Bits.Dirty) {
> -    PleB->Bits.Dirty = Attribute->Bits.Dirty;
> +    LocalPleB.Bits.Dirty = Attribute->Bits.Dirty;
>    }
>  
>    if (Mask->Bits.Pat) {
> -    PleB->Bits.Pat = Attribute->Bits.Pat;
> +    LocalPleB.Bits.Pat = Attribute->Bits.Pat;
>    }
>  
>    if (Mask->Bits.Global) {
> -    PleB->Bits.Global = Attribute->Bits.Global;
> +    LocalPleB.Bits.Global = Attribute->Bits.Global;
>    }
>  
>    if (Mask->Bits.ProtectionKey) {
> -    PleB->Bits.ProtectionKey = Attribute->Bits.ProtectionKey;
> +    LocalPleB.Bits.ProtectionKey = Attribute->Bits.ProtectionKey;
>    }
>  
>    if (Mask->Bits.Nx) {
> -    PleB->Bits.Nx = Attribute->Bits.Nx;
> +    LocalPleB.Bits.Nx = Attribute->Bits.Nx;
> +  }
> +
> +  if (PleB->Uint64 != LocalPleB.Uint64) {
> +    PleB->Uint64 = LocalPleB.Uint64;
>    }
>  }
>  
> @@ -186,24 +200,27 @@ PageTableLibSetPnle (
>    IN IA32_MAP_ATTRIBUTE        *Mask
>    )
>  {
> +  IA32_PAGE_NON_LEAF_ENTRY  LocalPnle;
> +
> +  LocalPnle.Uint64 = Pnle->Uint64;
>    if (Mask->Bits.Present) {
> -    Pnle->Bits.Present = Attribute->Bits.Present;
> +    LocalPnle.Bits.Present = Attribute->Bits.Present;
>    }
>  
>    if (Mask->Bits.ReadWrite) {
> -    Pnle->Bits.ReadWrite = Attribute->Bits.ReadWrite;
> +    LocalPnle.Bits.ReadWrite = Attribute->Bits.ReadWrite;
>    }
>  
>    if (Mask->Bits.UserSupervisor) {
> -    Pnle->Bits.UserSupervisor = Attribute->Bits.UserSupervisor;
> +    LocalPnle.Bits.UserSupervisor = Attribute->Bits.UserSupervisor;
>    }
>  
>    if (Mask->Bits.Nx) {
> -    Pnle->Bits.Nx = Attribute->Bits.Nx;
> +    LocalPnle.Bits.Nx = Attribute->Bits.Nx;
>    }
>  
> -  Pnle->Bits.Accessed   = 0;
> -  Pnle->Bits.MustBeZero = 0;
> +  LocalPnle.Bits.Accessed   = 0;
> +  LocalPnle.Bits.MustBeZero = 0;
>  
>    //
>    // Set the attributes (WT, CD, A) to 0.
> @@ -211,8 +228,11 @@ PageTableLibSetPnle (
>    // So, it implictly requires PAT[0] is Write Back.
>    // Create a new parameter if caller requires to use a different memory type for accessing page directories.
>    //
> -  Pnle->Bits.WriteThrough  = 0;
> -  Pnle->Bits.CacheDisabled = 0;
> +  LocalPnle.Bits.WriteThrough  = 0;
> +  LocalPnle.Bits.CacheDisabled = 0;
> +  if (Pnle->Uint64 != LocalPnle.Uint64) {
> +    Pnle->Uint64 = LocalPnle.Uint64;
> +  }
>  }
>  
>  /**



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