Re: [PATCH v9] UefiCpuPkg\CpuSmm: Save & restore CR2 on-demand paging in SMM

[Laszlo Ersek <lersek@redhat.com>]

On 04/01/19 10:16, nkvangup wrote:
> BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=1593
> 
> For every SMI occurrence, save and restore CR2 register only when SMM
> on-demand paging support is enabled in 64 bit operation mode.
> This is not a bug but to have better improvement of code.
> 
> Patch5 is updated with separate functions for Save and Restore of CR2
> based on review feedback.
> 
> Patch6 - Removed Global Cr2 instead used function parameter.
> 
> Patch7 - Removed checking Cr2 with 0 as per feedback.
> 
> Patch8 and 9 - Aligned with EDK2 Coding style.
> 
> Contributed-under: TianoCore Contribution Agreement 1.1
> Signed-off-by: Vanguput Narendra K <narendra.k.vanguput@intel.com>
> Cc: Eric Dong <eric.dong@intel.com>
> Cc: Ray Ni <ray.ni@intel.com>
> Cc: Laszlo Ersek <lersek@redhat.com>
> Cc: Yao Jiewen <jiewen.yao@intel.com>
> ---
>  UefiCpuPkg/PiSmmCpuDxeSmm/Ia32/PageTbl.c   | 26 ++++++++++++++++++++++++++
>  UefiCpuPkg/PiSmmCpuDxeSmm/MpService.c      |  9 ++++++---
>  UefiCpuPkg/PiSmmCpuDxeSmm/PiSmmCpuDxeSmm.h | 22 ++++++++++++++++++++++
>  UefiCpuPkg/PiSmmCpuDxeSmm/X64/PageTbl.c    | 30 ++++++++++++++++++++++++++++++
>  4 files changed, 84 insertions(+), 3 deletions(-)
> 
> diff --git a/UefiCpuPkg/PiSmmCpuDxeSmm/Ia32/PageTbl.c b/UefiCpuPkg/PiSmmCpuDxeSmm/Ia32/PageTbl.c
> index b734a1ea8c..d1e146a70c 100644
> --- a/UefiCpuPkg/PiSmmCpuDxeSmm/Ia32/PageTbl.c
> +++ b/UefiCpuPkg/PiSmmCpuDxeSmm/Ia32/PageTbl.c
> @@ -316,3 +316,29 @@ SetPageTableAttributes (
>  
>    return ;
>  }
> +
> +/**
> +  This function returns with no action for 32 bit.
> +
> +  @param[out]  *Cr2  Pointer to variable to hold CR2 register value.
> +**/
> +VOID
> +SaveCr2 (
> +  OUT UINTN  *Cr2
> +  )
> +{
> +  return ;
> +}
> +
> +/**
> +  This function returns with no action for 32 bit.
> +
> +  @param[in]  Cr2  Value to write into CR2 register.
> +**/
> +VOID
> +RestoreCr2 (
> +  IN UINTN  Cr2
> +  )
> +{
> +  return ;
> +}
> diff --git a/UefiCpuPkg/PiSmmCpuDxeSmm/MpService.c b/UefiCpuPkg/PiSmmCpuDxeSmm/MpService.c
> index 3b0b3b52ac..ce70f77709 100644
> --- a/UefiCpuPkg/PiSmmCpuDxeSmm/MpService.c
> +++ b/UefiCpuPkg/PiSmmCpuDxeSmm/MpService.c
> @@ -1112,9 +1112,11 @@ SmiRendezvous (
>    ASSERT(CpuIndex < mMaxNumberOfCpus);
>  
>    //
> -  // Save Cr2 because Page Fault exception in SMM may override its value
> +  // Save Cr2 because Page Fault exception in SMM may override its value,
> +  // when using on-demand paging for above 4G memory.
>    //
> -  Cr2 = AsmReadCr2 ();
> +  Cr2 = 0;
> +  SaveCr2 (&Cr2);
>  
>    //
>    // Perform CPU specific entry hooks
> @@ -1253,10 +1255,11 @@ SmiRendezvous (
>  
>  Exit:
>    SmmCpuFeaturesRendezvousExit (CpuIndex);
> +
>    //
>    // Restore Cr2
>    //
> -  AsmWriteCr2 (Cr2);
> +  RestoreCr2 (Cr2);
>  }
>  
>  /**
> diff --git a/UefiCpuPkg/PiSmmCpuDxeSmm/PiSmmCpuDxeSmm.h b/UefiCpuPkg/PiSmmCpuDxeSmm/PiSmmCpuDxeSmm.h
> index 84efb22981..38f9104117 100644
> --- a/UefiCpuPkg/PiSmmCpuDxeSmm/PiSmmCpuDxeSmm.h
> +++ b/UefiCpuPkg/PiSmmCpuDxeSmm/PiSmmCpuDxeSmm.h
> @@ -1243,4 +1243,26 @@ EFIAPI
>  PiSmmCpuSmiEntryFixupAddress (
>   );
>  
> +/**
> +  This function reads CR2 register when on-demand paging is enabled
> +  for 64 bit and no action for 32 bit.
> +
> +  @param[out]  *Cr2  Pointer to variable to hold CR2 register value.
> +**/
> +VOID
> +SaveCr2 (
> +  OUT UINTN  *Cr2
> +  );
> +
> +/**
> +  This function writes into CR2 register when on-demand paging is enabled
> +  for 64 bit and no action for 32 bit.
> +
> +  @param[in]  Cr2  Value to write into CR2 register.
> +**/
> +VOID
> +RestoreCr2 (
> +  IN UINTN  Cr2
> +  );
> +
>  #endif
> diff --git a/UefiCpuPkg/PiSmmCpuDxeSmm/X64/PageTbl.c b/UefiCpuPkg/PiSmmCpuDxeSmm/X64/PageTbl.c
> index 2c77cb47a4..95eaf0b016 100644
> --- a/UefiCpuPkg/PiSmmCpuDxeSmm/X64/PageTbl.c
> +++ b/UefiCpuPkg/PiSmmCpuDxeSmm/X64/PageTbl.c
> @@ -1053,3 +1053,33 @@ SetPageTableAttributes (
>  
>    return ;
>  }
> +
> +/**
> +  This function reads CR2 register when on-demand paging is enabled.
> +
> +  @param[out]  *Cr2  Pointer to variable to hold CR2 register value.
> +**/
> +VOID
> +SaveCr2 (
> +  OUT UINTN  *Cr2
> +  )
> +{
> +  if (!mCpuSmmStaticPageTable) {
> +    *Cr2 = AsmReadCr2 ();
> +  }
> +}
> +
> +/**
> +  This function restores CR2 register when on-demand paging is enabled.
> +
> +  @param[in]  Cr2  Value to write into CR2 register.
> +**/
> +VOID
> +RestoreCr2 (
> +  IN UINTN  Cr2
> +  )
> +{
> +  if (!mCpuSmmStaticPageTable) {
> +    AsmWriteCr2 (Cr2);
> +  }
> +}
> 

I agree *how* this patch is implemented is correct, wrt. the IA32 / X64
split.

A slight improvement for edk2 coding style would be to replace "*Cr2"
with just "Cr2" in the @param[out] comments, but there's no need to
repost the patch just because of that.

Regarding the "what" and "why", Nate's and Andrew's comments under v8
make me uncomfortable about the patch. While the pre-patch comments do say

  Save Cr2 because Page Fault exception in SMM may override its value

the post-patch comment (and code) are more restricted -- they claim that
such an exception (from which we return, anyway) may only occur when
on-demand paging is enabled (which is in turn a pre-requisite to both
the SMM profile feature and the SMM heap guard feature).

It is this "narrowing" that concerns me (i.e. the claim that a page
fault that we consider "expected", and return from, may only occur due
to enabling on-demand paging). It *seems* like a correct statement, but
I'd like other reviewers to prove (or disprove) it; so I will not give
either A-b or R-b.

On the testing front, I confirm the patch doesn't regress OVMF. (OVMF
has on-demand paging *disabled* -- it uses static page tables in X64 SMM
--, so there the patch removes the CR2 save/restore, on both IA32 and X64.)

Regression-tested-by: Laszlo Ersek <lersek@redhat.com>

Thanks
Laszlo