Re: [Patch] UefiCpuPkg/MpLib: Fix PEI Services Table pointer on AP

[Felix Polyudov <Felixp@ami.com>]

Ray,

I agree with the premise that calling PEI services from AP should generally be avoided.
However, the PEI services can be used on AP under certain special circumstances.
A couple of examples:
1. For debugging purposes. The MpInitLib contains 12 DEBUG calls and 19 ASSERT calls. Depending on the DebugLib instance used in the project, these calls may lead to PEI services invocation.
2. MpInitLib provides ability to call AP in a serialized manner (only one AP is running, other APs and BSP are waiting), when it is safe to call PEI services.

Additionally, I think even if PEI services should not be used on AP, there is still a reason to keep PEI services table pointer initialized.
On one hand, given the complexity of modern firmware projects comprised of modules coming from multiple vendors, making sure PEI services are not used on AP can be a challenge.
For example, in my case the call was coming from the chipset reference code.
On the other hand, with the current implementation, when somebody does try to use PEI services on AP the behavior is unpredictable.
Depending on the content of the uninitialized PEI service table pointer, the system may either crash with one of the handful of different exceptions,
or it may start executing code from a random location. It's very difficult to debug such issues. One can spend weeks chasing a problem like this.


-----Original Message-----
From: edk2-devel [mailto:edk2-devel-bounces@lists.01.org] On Behalf Of Ni, Ruiyu
Sent: Thursday, November 29, 2018 10:43 PM
To: Felix Polyudov; edk2-devel@lists.01.org
Cc: lersek@redhat.com; Dong, Eric
Subject: Re: [edk2] [Patch] UefiCpuPkg/MpLib: Fix PEI Services Table pointer on AP

Felix,
I disagree:) Sorry about that. :)

The commit you mentioned might be made by me (didn't checked).
Because I aimed to avoid calling PEI services from AP. That's a violation of PI spec and not safe by design.

The AP calling standard services concern was raised by Andrew initially.

Thanks,
Ray

> -----Original Message-----
> From: Felix Polyudov [mailto:felixp@ami.com]
> Sent: Friday, November 30, 2018 8:36 AM
> To: edk2-devel@lists.01.org
> Cc: Dong, Eric <eric.dong@intel.com>; Ni, Ruiyu <ruiyu.ni@intel.com>;
> lersek@redhat.com
> Subject: [Patch] UefiCpuPkg/MpLib: Fix PEI Services Table pointer on AP
>
> According to PI specification PEI Services table pointer is stored right before ITD
> base. Starting from commit c563077a380437c1 BSP and AP have different IDT
> instances.
> PEI Services table pointer was not initialized in the AP IDT instance.
> As a result, any attempt to use functions from PeiServicesTablePointerLib or
> PeiServicesLib on AP caused CPU exception.
>
> Contributed-under: TianoCore Contribution Agreement 1.1
> Signed-off-by: Felix Polyudov <felixp@ami.com>
> ---
>  UefiCpuPkg/Library/MpInitLib/MpLib.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/UefiCpuPkg/Library/MpInitLib/MpLib.c
> b/UefiCpuPkg/Library/MpInitLib/MpLib.c
> index 7f4d6e6..0e3e362 100644
> --- a/UefiCpuPkg/Library/MpInitLib/MpLib.c
> +++ b/UefiCpuPkg/Library/MpInitLib/MpLib.c
> @@ -1567,6 +1567,7 @@ MpInitLibInitialize (
>    BufferSize  = ApStackSize * MaxLogicalProcessorNumber;
>    BufferSize += MonitorFilterSize * MaxLogicalProcessorNumber;
>    BufferSize += ApResetVectorSize;
> +  BufferSize += sizeof(UINTN);
>    BufferSize  = ALIGN_VALUE (BufferSize, 8);
>    BufferSize += VolatileRegisters.Idtr.Limit + 1;
>    BufferSize += sizeof (CPU_MP_DATA);
> @@ -1587,6 +1588,8 @@ MpInitLibInitialize (
>    //         Backup Buffer
>    //    +--------------------+
>    //           Padding
> +  //    +--------------------+
> +  //    PEI Services Table Pointer
>    //    +--------------------+ <-- ApIdtBase (8-byte boundary)
>    //           AP IDT          All APs share one separate IDT. So AP can get address of
> CPU_MP_DATA from IDT Base.
>    //    +--------------------+ <-- CpuMpData
> @@ -1599,7 +1602,7 @@ MpInitLibInitialize (
>    //
>    MonitorBuffer    = (UINT8 *) (Buffer + ApStackSize *
> MaxLogicalProcessorNumber);
>    BackupBufferAddr = (UINTN) MonitorBuffer + MonitorFilterSize *
> MaxLogicalProcessorNumber;
> -  ApIdtBase        = ALIGN_VALUE (BackupBufferAddr + ApResetVectorSize, 8);
> +  ApIdtBase        = ALIGN_VALUE (BackupBufferAddr + ApResetVectorSize +
> sizeof(UINTN), 8);
>    CpuMpData        = (CPU_MP_DATA *) (ApIdtBase + VolatileRegisters.Idtr.Limit +
> 1);
>    CpuMpData->Buffer           = Buffer;
>    CpuMpData->CpuApStackSize   = ApStackSize;
> @@ -1653,6 +1656,11 @@ MpInitLibInitialize (
>            Buffer + BufferSize);
>
>    //
> +  // Initialize PEI Services table pointer. Copy the address from BSP.
> +  //
> +  *(UINTN*)(ApIdtBase - sizeof(UINTN)) =
> + *(UINTN*)(VolatileRegisters.Idtr.Base - sizeof (UINTN));
> +
> +  //
>    // Duplicate BSP's IDT to APs.
>    // All APs share one separate IDT. So AP can get the address of CpuMpData by
> using IDTR.BASE + IDTR.LIMIT + 1
>    //
> --
> 2.10.0.windows.1
>
>
>
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