Re: [edk2-devel] [Patch V2] UefiCpuPkg/CpuExceptionHandlerLib: Fix #AC split lock

["Dong, Eric" <eric.dong@intel.com>]

Cc other reviewers.

Reviewed-by: Eric Dong <eric.dong@intel.com>

Thanks,
Eric
> -----Original Message-----
> From: devel@edk2.groups.io <devel@edk2.groups.io> On Behalf Of John E
> Lofgren
> Sent: Tuesday, September 10, 2019 2:41 AM
> To: devel@edk2.groups.io
> Subject: [edk2-devel] [Patch V2] UefiCpuPkg/CpuExceptionHandlerLib: Fix
> #AC split lock
> 
> REF: https://bugzilla.tianocore.org/show_bug.cgi?id=2150
> 
> V2 changes:
> Add xchg 16 bit instructions to handle sgdt and sidt base
> 63:48 bits and 47:32 bits.
> Add comment to explain why xchg 64bit isnt being used
> 
> Fix #AC split lock's caused by seperating base and limit from sgdt and sidt by
> changing xchg operands to 32-bit to stop from crossing cacheline.
> 
> Signed-off-by: John E Lofgren <john.e.lofgren@intel.com>
> ---
> 
> UefiCpuPkg/Library/CpuExceptionHandlerLib/X64/ExceptionHandlerAsm.nas
> m | 20 ++++++++++++++------
>  1 file changed, 14 insertions(+), 6 deletions(-)
> 
> diff --git
> a/UefiCpuPkg/Library/CpuExceptionHandlerLib/X64/ExceptionHandlerAsm.n
> asm
> b/UefiCpuPkg/Library/CpuExceptionHandlerLib/X64/ExceptionHandlerAsm.n
> asm
> index 4db1a09f28..7b7642b290 100644
> ---
> a/UefiCpuPkg/Library/CpuExceptionHandlerLib/X64/ExceptionHandlerAsm.n
> asm
> +++
> b/UefiCpuPkg/Library/CpuExceptionHandlerLib/X64/ExceptionHandlerAsm.
> +++ nasm
> @@ -180,21 +180,29 @@ HasErrorCode:
>      push    qword [rbp + 24]
> 
>  ;; UINT64  Gdtr[2], Idtr[2];
> +    ; sidt and sgdt saves 10 bytes to memory, 8 bytes = base and 2 bytes =
> limit.
> +    ; To avoid #AC split lock when separating base and limit into their
> +    ; own separate 64 bit memory, we can’t use 64 bit xchg since base
> [63:48] bits
> +    ; may cross the cache line.
>      xor     rax, rax
>      push    rax
>      push    rax
>      sidt    [rsp]
> -    xchg    rax, [rsp + 2]
> -    xchg    rax, [rsp]
> -    xchg    rax, [rsp + 8]
> +    xchg    eax, [rsp + 2]
> +    xchg    eax, [rsp]
> +    xchg    eax, [rsp + 8]
> +    xchg     ax, [rsp + 6]
> +    xchg     ax, [rsp + 4]
> 
>      xor     rax, rax
>      push    rax
>      push    rax
>      sgdt    [rsp]
> -    xchg    rax, [rsp + 2]
> -    xchg    rax, [rsp]
> -    xchg    rax, [rsp + 8]
> +    xchg    eax, [rsp + 2]
> +    xchg    eax, [rsp]
> +    xchg    eax, [rsp + 8]
> +    xchg     ax, [rsp + 6]
> +    xchg     ax, [rsp + 4]
> 
>  ;; UINT64  Ldtr, Tr;
>      xor     rax, rax
> --
> 2.16.2.windows.1
> 
> 
>