Re: [PATCH] UefiCpuPkg/MpInitLib: fix wrong base address set as Stack Guard

[Laszlo Ersek <lersek@redhat.com>]

(CC Jeff)

Sorry about the delay.

I have some light comments below; I expect at least a few of them to be
incorrect :)

On 12/29/17 09:36, Jian J Wang wrote:
> The reason is that DXE part initialization will reuse the stack allocated
> at PEI phase, if MP was initialized before. Some code added to check this
> situation and use stack base address saved in HOB passed from PEI.
> 
> Cc: Jiewen Yao <jiewen.yao@intel.com>
> Cc: Eric Dong <eric.dong@intel.com>
> Cc: Laszlo Ersek <lersek@redhat.com>
> Contributed-under: TianoCore Contribution Agreement 1.1
> Signed-off-by: Jian J Wang <jian.j.wang@intel.com>
> ---
>  UefiCpuPkg/Library/MpInitLib/DxeMpLib.c | 17 +++++++++++++++--
>  1 file changed, 15 insertions(+), 2 deletions(-)
> 
> diff --git a/UefiCpuPkg/Library/MpInitLib/DxeMpLib.c b/UefiCpuPkg/Library/MpInitLib/DxeMpLib.c
> index 40c1bf407a..05484c9ff3 100644
> --- a/UefiCpuPkg/Library/MpInitLib/DxeMpLib.c
> +++ b/UefiCpuPkg/Library/MpInitLib/DxeMpLib.c
> @@ -295,6 +295,7 @@ InitMpGlobalData (
>    UINTN                               Index;
>    EFI_GCD_MEMORY_SPACE_DESCRIPTOR     MemDesc;
>    UINTN                               StackBase;
> +  CPU_INFO_IN_HOB                     *CpuInfoInHob;
>  
>    SaveCpuMpData (CpuMpData);
>  
> @@ -314,9 +315,18 @@ InitMpGlobalData (
>        ASSERT (FALSE);
>      }
>  
> -    for (Index = 0; Index < CpuMpData->CpuCount; ++Index) {
> -      StackBase = CpuMpData->Buffer + Index * CpuMpData->CpuApStackSize;
> +    //
> +    // DXE will reuse stack allocated for APs at PEI phase if it's available.
> +    // Let's check it here.
> +    //
> +    CpuInfoInHob = (CPU_INFO_IN_HOB *)(UINTN)CpuMpData->CpuInfoInHob;
> +    if (CpuInfoInHob != NULL && CpuInfoInHob->ApTopOfStack != 0) {
> +      StackBase = CpuInfoInHob->ApTopOfStack;
> +    } else {
> +      StackBase = CpuMpData->Buffer;
> +    }

So, if the HOB is not found, then StackBase is set okay.

However, I'm unsure about the other case. The
CPU_INFO_IN_HOB.ApTopOfStack field identifies the *top* of the stack
(highest address, and the stack grows down); however the loop below
*increments* StackBase. Given the incrementing nature of the loop,
shouldn't we first calculate the actual base (= lowest address) from the
CPU_INFO_IN_HOB.ApTopOfStack field?

Actually... I'm even more confused. The CpuMpData->CpuInfoInHob field
points to an *array* of CPU_INFO_IN_HOB structures. Therefore, for any
given processor #N, we should not calculate the stack base as

  CpuMpData->CpuInfoInHob->ApTopOfStack + N * CpuMpData->CpuApStackSize

instead we should calculate the stack base as something like:

  CpuMpData->CpuInfoInHob[N].ApTopOfStack - CpuMpData->CpuApStackSize

See
- the InitializeApData() function,
- and its call site in the ApWakeupFunction() function.

(To my surprise, I personally modified InitializeApData() earlier, in
commit dd3fa0cd72de ("UefiCpuPkg/MpInitLib: support 64-bit AP stack
addresses", 2016-11-17) -- I've totally forgotten about that by now!)

What do you think?

>  
> +    for (Index = 0; Index < CpuMpData->CpuCount; ++Index) {
>        Status = gDS->GetMemorySpaceDescriptor (StackBase, &MemDesc);
>        ASSERT_EFI_ERROR (Status);
>  
> @@ -326,6 +336,9 @@ InitMpGlobalData (
>                        MemDesc.Attributes | EFI_MEMORY_RP
>                        );
>        ASSERT_EFI_ERROR (Status);
> +
> +      DEBUG ((DEBUG_VERBOSE, "Stack Guard set at %x [cpu%d]!\n", StackBase, Index));

StackBase has type UINTN, and so it should not be printed with %x. It
should be cast to (UINT64), and then printed with %Lx.

Similarly, Index has type UINTN. It should not be printed with %d. It
should be cast to (UINT64) and printed with %Lu.


> +      StackBase += CpuMpData->CpuApStackSize;

Again, I don't think the simple increment applies when the
CpuMpData->CpuInfoInHob array exists.

>      }
>    }
>  
> 

Thanks,
Laszlo