Re: [edk2-devel] [Patch V3] UefiCpuPkg/CpuExceptionHandlerLib: Fix split lock

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

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

And pushed:
  SHA-1: f4c898f2b2db2819c519cdce05403d4ba0234979

Thanks,
Eric

> -----Original Message-----
> From: devel@edk2.groups.io [mailto:devel@edk2.groups.io] On Behalf Of John
> E Lofgren
> Sent: Wednesday, September 18, 2019 11:43 PM
> To: devel@edk2.groups.io
> Subject: [edk2-devel] [Patch V3] UefiCpuPkg/CpuExceptionHandlerLib: Fix split
> lock
> 
> REF: https://bugzilla.tianocore.org/show_bug.cgi?id=2150
> V3 changes:
> change to mov instruction (non locking instuction) instead
> of xchg to simplify design.
> 
> 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
> 
> Split lock happens when a locking instruction is used on mis-aligned data
> that crosses two cachelines. If close source platform enables Alignment Check
> Exception(#AC), They can hit a double fault due to split lock being in
> CpuExceptionHandlerLib.
> 
> sigt and sgdt saves 10 bytes to memory, 8 bytes is base and 2 bytes is limit.
> The data is mis-aligned, can cross two cacheline, and a xchg
> instruction(locking instuction) is being utilize.
> 
> Signed-off-by: John E Lofgren <john.e.lofgren@intel.com>
> ---
>  UefiCpuPkg/Library/CpuExceptionHandlerLib/X64/ExceptionHandlerAsm.nasm
> | 14 ++++++++------
>  1 file changed, 8 insertions(+), 6 deletions(-)
> 
> diff --git
> a/UefiCpuPkg/Library/CpuExceptionHandlerLib/X64/ExceptionHandlerAsm.nas
> m
> b/UefiCpuPkg/Library/CpuExceptionHandlerLib/X64/ExceptionHandlerAsm.nas
> m
> index 4db1a09f28..19198f2731 100644
> ---
> a/UefiCpuPkg/Library/CpuExceptionHandlerLib/X64/ExceptionHandlerAsm.nas
> m
> +++
> b/UefiCpuPkg/Library/CpuExceptionHandlerLib/X64/ExceptionHandlerAsm.nas
> m
> @@ -184,17 +184,19 @@ HasErrorCode:
>      push    rax
>      push    rax
>      sidt    [rsp]
> -    xchg    rax, [rsp + 2]
> -    xchg    rax, [rsp]
> -    xchg    rax, [rsp + 8]
> +    mov     bx, word [rsp]
> +    mov     rax, qword [rsp + 2]
> +    mov     qword [rsp], rax
> +    mov     word [rsp + 8], bx
> 
>      xor     rax, rax
>      push    rax
>      push    rax
>      sgdt    [rsp]
> -    xchg    rax, [rsp + 2]
> -    xchg    rax, [rsp]
> -    xchg    rax, [rsp + 8]
> +    mov     bx, word [rsp]
> +    mov     rax, qword [rsp + 2]
> +    mov     qword [rsp], rax
> +    mov     word [rsp + 8], bx
> 
>  ;; UINT64  Ldtr, Tr;
>      xor     rax, rax
> --
> 2.16.2.windows.1
> 
> 
>