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>  I believe something like that would do:
>
> CpuExceptionHandlerLib supplies misaligned GDT to the outer world
> (e.g. ArchSetupExceptionStack) when PcdCpuStackGuard is enabled.
> This happens because it uses an array of UINT8 for the mNewGdt
> variable, which alignment is 1 byte versus required 8 bytes. As a result
> ArchSetupExceptionStack always returns EFI_INVALID_PARAMETER in OVMF Ia32
> with XCODE5 and CLANGPDB at least.
>
> Fix this by allocating extra space in mNewGdt and then aligning the pointer
> upwards.

But I'm happy with this one.

Best Regards,

Leif

> Best wishes,
> Vitaly
>
> > On 5 Nov 2021, at 22:28, Leif Lindholm <leif@nuviainc.com> wrote:
> >
> > UefiCpuPkg maintainers - please respond.
> >
> > Meanwhile, Vitaly, could you please provide a commit message?
> > The BZ link is needed, but it's not a substitute.
> >
> > /
> >    Leif
> >
> > On Mon, Sep 20, 2021 at 17:13:47 +0300, Vitaly Cheptsov wrote:
> >> REF: https://bugzilla.tianocore.org/show_bug.cgi?id=3639
> >>
> >>
> >>
> >> Cc: Jiewen Yao <jiewen.yao@intel.com>
> >>
> >> Cc: Eric Dong <eric.dong@intel.com>
> >>
> >> Cc: Michael Kinney <michael.d.kinney@intel.com>
> >>
> >> Cc: Jian J Wang <jian.j.wang@intel.com>
> >>
> >> Cc: Jeff Fan <vanjeff_919@hotmail.com>
> >>
> >> Cc: Mikhail Krichanov <krichanov@ispras.ru>
> >>
> >> Cc: Marvin Häuser <mhaeuser@posteo.de>
> >>
> >> Signed-off-by: Vitaly Cheptsov <cheptsov@ispras.ru>
> >>
> >> ---
> >>
> >> .../Library/CpuExceptionHandlerLib/DxeException.c    | 12 +++++++-----
> >>
> >> 1 file changed, 7 insertions(+), 5 deletions(-)
> >>
> >>
> >>
> >> diff --git a/UefiCpuPkg/Library/CpuExceptionHandlerLib/DxeException.c b/UefiCpuPkg/Library/CpuExceptionHandlerLib/DxeException.c
> >>
> >> index fd59f09ecd..12874811e1 100644
> >>
> >> --- a/UefiCpuPkg/Library/CpuExceptionHandlerLib/DxeException.c
> >>
> >> +++ b/UefiCpuPkg/Library/CpuExceptionHandlerLib/DxeException.c
> >>
> >> @@ -22,7 +22,7 @@ EXCEPTION_HANDLER_DATA      mExceptionHandlerData;
> >>
> >>
> >>
> >> UINT8                       mNewStack[CPU_STACK_SWITCH_EXCEPTION_NUMBER *
> >>
> >>                                       CPU_KNOWN_GOOD_STACK_SIZE];
> >>
> >> -UINT8                       mNewGdt[CPU_TSS_GDT_SIZE];
> >>
> >> +UINT8                       mNewGdt[CPU_TSS_GDT_SIZE + IA32_GDT_ALIGNMENT];
> >>
> >>
> >>
> >> /**
> >>
> >>   Common exception handler.
> >>
> >> @@ -238,6 +238,7 @@ InitializeCpuExceptionHandlersEx (
> >>
> >>   CPU_EXCEPTION_INIT_DATA           EssData;
> >>
> >>   IA32_DESCRIPTOR                   Idtr;
> >>
> >>   IA32_DESCRIPTOR                   Gdtr;
> >>
> >> +  UINT8                             *Gdt;
> >>
> >>
> >>
> >>   //
> >>
> >>   // To avoid repeat initialization of default handlers, the caller should pass
> >>
> >> @@ -259,6 +260,7 @@ InitializeCpuExceptionHandlersEx (
> >>
> >>     if (PcdGetBool (PcdCpuStackGuard)) {
> >>
> >>       if (InitData == NULL) {
> >>
> >>         SetMem (mNewGdt, sizeof (mNewGdt), 0);
> >>
> >> +        Gdt = ALIGN_POINTER (mNewGdt, IA32_GDT_ALIGNMENT);
> >>
> >>
> >>
> >>         AsmReadIdtr (&Idtr);
> >>
> >>         AsmReadGdtr (&Gdtr);
> >>
> >> @@ -270,11 +272,11 @@ InitializeCpuExceptionHandlersEx (
> >>
> >>         EssData.X64.StackSwitchExceptionNumber = CPU_STACK_SWITCH_EXCEPTION_NUMBER;
> >>
> >>         EssData.X64.IdtTable = (VOID *)Idtr.Base;
> >>
> >>         EssData.X64.IdtTableSize = Idtr.Limit + 1;
> >>
> >> -        EssData.X64.GdtTable = mNewGdt;
> >>
> >> -        EssData.X64.GdtTableSize = sizeof (mNewGdt);
> >>
> >> -        EssData.X64.ExceptionTssDesc = mNewGdt + Gdtr.Limit + 1;
> >>
> >> +        EssData.X64.GdtTable = Gdt;
> >>
> >> +        EssData.X64.GdtTableSize = CPU_TSS_GDT_SIZE;
> >>
> >> +        EssData.X64.ExceptionTssDesc = Gdt + Gdtr.Limit + 1;
> >>
> >>         EssData.X64.ExceptionTssDescSize = CPU_TSS_DESC_SIZE;
> >>
> >> -        EssData.X64.ExceptionTss = mNewGdt + Gdtr.Limit + 1 + CPU_TSS_DESC_SIZE;
> >>
> >> +        EssData.X64.ExceptionTss = Gdt + Gdtr.Limit + 1 + CPU_TSS_DESC_SIZE;
> >>
> >>         EssData.X64.ExceptionTssSize = CPU_TSS_SIZE;
> >>
> >>
> >>
> >>         InitData = &EssData;
> >>
> >> --
> >>
> >> 2.30.1 (Apple Git-130)
> >>
> >>
> >>
> >>
> >>
> >>
> >>
> >>