Re: [edk2-devel] [PATCH] UefiCpuPkg/MpInitLib: Allocate a separate SEV-ES AP reset stack area

["Marvin Häuser" <mhaeuser@posteo.de>]

Good day Thomas,

Thank you very much for the quick patch. Comments inline.

Best regards,
Marvin

On 11.05.21 22:50, Lendacky, Thomas wrote:
> BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=3324
>
> The SEV-ES stacks currently share a page with the reset code and data.
> Separate the SEV-ES stacks from the reset vector code and data to avoid
> possible stack overflows from overwriting the code and/or data.
>
> When SEV-ES is enabled, invoke the GetWakeupBuffer() routine a second time
> to allocate a new area, below the reset vector and data.
>
> Both the PEI and DXE versions of GetWakeupBuffer() are changed to track
> the previous reset buffer allocation in order to ensure that the new
> buffer allocation is below the previous allocation.
>
> Fixes: 7b7508ad784d16a5208c8d12dff43aef6df0835b
> Cc: Eric Dong <eric.dong@intel.com>
> Cc: Ray Ni <ray.ni@intel.com>
> Cc: Laszlo Ersek <lersek@redhat.com>
> Cc: Rahul Kumar <rahul1.kumar@intel.com>
> Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
> ---
>   UefiCpuPkg/Library/MpInitLib/DxeMpLib.c | 12 ++++-
>   UefiCpuPkg/Library/MpInitLib/MpLib.c    | 48 +++++++++++++-------
>   UefiCpuPkg/Library/MpInitLib/PeiMpLib.c | 14 ++++--
>   3 files changed, 54 insertions(+), 20 deletions(-)
>
> diff --git a/UefiCpuPkg/Library/MpInitLib/DxeMpLib.c b/UefiCpuPkg/Library/MpInitLib/DxeMpLib.c
> index 7839c249760e..fdfa0755d37a 100644
> --- a/UefiCpuPkg/Library/MpInitLib/DxeMpLib.c
> +++ b/UefiCpuPkg/Library/MpInitLib/DxeMpLib.c
> @@ -29,6 +29,11 @@ VOID             *mReservedApLoopFunc = NULL;
>   UINTN            mReservedTopOfApStack;
>   volatile UINT32  mNumberToFinish = 0;
>   
> +//
> +// Begin wakeup buffer allocation below 0x88000
> +//

This definitely is not an issue of your patch at all, but I find the 
comments of the behaviour very lacking. Might this be a good opportunity 
to briefly document it? It took me quite a bit of git blame to fully 
figure it out, especially due to the non-descriptive commit message. The 
constant is explained very well in the description: 
https://github.com/tianocore/edk2/commit/e4ff6349bf9ee4f3f392141374901ea4994e043e

> +STATIC EFI_PHYSICAL_ADDRESS mWakeupBuffer = 0x88000;

Hmm, I wonder whether a global variable is the best solution here. With 
an explanation from the commit above, a top-down allocator makes good 
sense for this scenario. However, a "GetWakeupBuffer(Size, MaxAddress)" 
function might be more self-descriptive. What do you think?

> +
>   /**
>     Enable Debug Agent to support source debugging on AP function.
>   
> @@ -102,7 +107,7 @@ GetWakeupBuffer (
>     // LagacyBios driver depends on CPU Arch protocol which guarantees below
>     // allocation runs earlier than LegacyBios driver.
>     //
> -  StartAddress = 0x88000;
> +  StartAddress = mWakeupBuffer;
>     Status = gBS->AllocatePages (
>                     AllocateMaxAddress,
>                     MemoryType,
> @@ -112,6 +117,11 @@ GetWakeupBuffer (
>     ASSERT_EFI_ERROR (Status);
>     if (EFI_ERROR (Status)) {
>       StartAddress = (EFI_PHYSICAL_ADDRESS) -1;
> +  } else {
> +    //
> +    // Next wakeup buffer allocation must be below this allocation
> +    //
> +    mWakeupBuffer = StartAddress;
>     }
>   
>     DEBUG ((DEBUG_INFO, "WakeupBufferStart = %x, WakeupBufferSize = %x\n",
> diff --git a/UefiCpuPkg/Library/MpInitLib/MpLib.c b/UefiCpuPkg/Library/MpInitLib/MpLib.c
> index dc2a54aa31e8..a76dae437606 100644
> --- a/UefiCpuPkg/Library/MpInitLib/MpLib.c
> +++ b/UefiCpuPkg/Library/MpInitLib/MpLib.c
> @@ -1164,20 +1164,6 @@ GetApResetVectorSize (
>              AddressMap->SwitchToRealSize +
>              sizeof (MP_CPU_EXCHANGE_INFO);
>   
> -  //
> -  // The AP reset stack is only used by SEV-ES guests. Do not add to the
> -  // allocation if SEV-ES is not enabled.
> -  //
> -  if (PcdGetBool (PcdSevEsIsEnabled)) {
> -    //
> -    // Stack location is based on APIC ID, so use the total number of
> -    // processors for calculating the total stack area.
> -    //
> -    Size += AP_RESET_STACK_SIZE * PcdGet32 (PcdCpuMaxLogicalProcessorNumber);
> -
> -    Size = ALIGN_VALUE (Size, CPU_STACK_ALIGNMENT);
> -  }
> -
>     return Size;
>   }
>   
> @@ -1207,9 +1193,39 @@ AllocateResetVector (
>                                       CpuMpData->AddressMap.ModeTransitionOffset
>                                       );
>       //
> -    // The reset stack starts at the end of the buffer.
> +    // The AP reset stack is only used by SEV-ES guests. Do not allocate it
> +    // if SEV-ES is not enabled.
>       //
> -    CpuMpData->SevEsAPResetStackStart = CpuMpData->WakeupBuffer + ApResetVectorSize;
> +    if (PcdGetBool (PcdSevEsIsEnabled)) {
> +      UINTN  ApResetStackSize;

Personally, I do not mind this at all, but I think the code style 
prohibits declaring variables in inner scopes. Though preferably I would 
like to see this, nowadays, arbitrary restriction lifted rather than the 
patch be changed. Do the package maintainers have comments on this?

> +
> +      //
> +      // Stack location is based on ProcessorNumber, so use the total number
> +      // of processors for calculating the total stack area.
> +      //
> +      ApResetStackSize = AP_RESET_STACK_SIZE *
> +                           PcdGet32 (PcdCpuMaxLogicalProcessorNumber);
> +
> +      //
> +      // Invoke GetWakeupBuffer a second time to allocate the stack area
> +      // below 1MB. The returned buffer will be page aligned and sized and
> +      // below the previously allocated buffer.
> +      //
> +      CpuMpData->SevEsAPResetStackStart = GetWakeupBuffer (ApResetStackSize);
> +
> +      //
> +      // Check to be sure that the "allocate below" behavior hasn't changed.
> +      // This will also catch a failed allocation, as "-1" is returned on
> +      // failure.
> +      //
> +      if (CpuMpData->SevEsAPResetStackStart >= CpuMpData->WakeupBuffer) {
> +        DEBUG ((DEBUG_ERROR,
> +          "SEV-ES AP reset stack is not below wakeup buffer\n"));
> +
> +        ASSERT (FALSE);

Should the ASSERT not only catch the broken "allocate below" behaviour, 
i.e. not trigger on failed allocation?

> +        CpuDeadLoop ();
> +      }
> +    }
>     }
>     BackupAndPrepareWakeupBuffer (CpuMpData);
>   }
> diff --git a/UefiCpuPkg/Library/MpInitLib/PeiMpLib.c b/UefiCpuPkg/Library/MpInitLib/PeiMpLib.c
> index 3989bd6a7a9f..4d09e89b4128 100644
> --- a/UefiCpuPkg/Library/MpInitLib/PeiMpLib.c
> +++ b/UefiCpuPkg/Library/MpInitLib/PeiMpLib.c
> @@ -11,6 +11,8 @@
>   #include <Guid/S3SmmInitDone.h>
>   #include <Ppi/ShadowMicrocode.h>
>   
> +STATIC UINT64 mWakeupBuffer = BASE_1MB;
> +
>   /**
>     S3 SMM Init Done notification function.
>   
> @@ -220,11 +222,11 @@ GetWakeupBuffer (
>           // Need memory under 1MB to be collected here
>           //
>           WakeupBufferEnd = Hob.ResourceDescriptor->PhysicalStart + Hob.ResourceDescriptor->ResourceLength;
> -        if (WakeupBufferEnd > BASE_1MB) {
> +        if (WakeupBufferEnd > mWakeupBuffer) {
>             //
> -          // Wakeup buffer should be under 1MB
> +          // Wakeup buffer should be under 1MB and under the previous one
>             //
> -          WakeupBufferEnd = BASE_1MB;
> +          WakeupBufferEnd = mWakeupBuffer;
>           }
>           while (WakeupBufferEnd > WakeupBufferSize) {
>             //
> @@ -244,6 +246,12 @@ GetWakeupBuffer (
>             }
>             DEBUG ((DEBUG_INFO, "WakeupBufferStart = %x, WakeupBufferSize = %x\n",
>                                  WakeupBufferStart, WakeupBufferSize));
> +
> +          //
> +          // Next wakeup buffer allocation must be below this allocation
> +          //
> +          mWakeupBuffer = WakeupBufferStart;
> +
>             return (UINTN)WakeupBufferStart;
>           }
>         }