Re: [edk2-devel] [PATCH v3 1/6] OvmfPkg/ResetVector: convert SEV-ES Reset Block structure to be GUIDed

["Laszlo Ersek" <lersek@redhat.com>]

On 11/30/20 21:28, James Bottomley wrote:
> Convert the current ES reset block structure to an extensible guid
> based structure by appending a header and length, which allow for
> multiple guid based data packets to be inserted.
> 
> Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=3077
> Signed-off-by: James Bottomley <jejb@linux.ibm.com>
> 
> ---
> 
> v2: added
> v3: reworked
> ---
>  OvmfPkg/ResetVector/Ia16/ResetVectorVtf0.asm | 51 +++++++++++++++-----
>  1 file changed, 40 insertions(+), 11 deletions(-)

Reviewed-by: Laszlo Ersek <lersek@redhat.com>

Thank you!
Laszlo

> diff --git a/OvmfPkg/ResetVector/Ia16/ResetVectorVtf0.asm b/OvmfPkg/ResetVector/Ia16/ResetVectorVtf0.asm
> index 980e0138e7fe..9e0a74fddfc1 100644
> --- a/OvmfPkg/ResetVector/Ia16/ResetVectorVtf0.asm
> +++ b/OvmfPkg/ResetVector/Ia16/ResetVectorVtf0.asm
> @@ -25,21 +25,40 @@ ALIGN   16
>      TIMES (0x1000 - ($ - EndOfPageTables) - 0x20) DB 0
>  %endif
>  
> +;
> +; Padding to ensure first guid starts at 0xffffffd0
> +;
> +TIMES (15 - ((guidedStructureEnd - guidedStructureStart + 15) % 16)) DB 0
> +
> +; GUIDed structure.  To traverse this you should first verify the
> +; presence of the table footer guid
> +; (96b582de-1fb2-45f7-baea-a366c55a082d) at 0xffffffd0.  If that
> +; is found, the two bytes at 0xffffffce are the entire table length.
> +;
> +; The table is composed of structures with the form:
> +;
> +; Data (arbitrary bytes identified by guid)
> +; length from start of data to end of guid (2 bytes)
> +; guid (16 bytes)
> +;
> +; so work back from the footer using the length to traverse until you
> +; either find the guid you're looking for or run off the beginning of
> +; the table.
> +;
> +guidedStructureStart:
> +
>  ;
>  ; SEV-ES Processor Reset support
>  ;
>  ; sevEsResetBlock:
>  ;   For the initial boot of an AP under SEV-ES, the "reset" RIP must be
> -;   programmed to the RAM area defined by SEV_ES_AP_RESET_IP. A known offset
> -;   and GUID will be used to locate this block in the firmware and extract
> -;   the build time RIP value. The GUID must always be 48 bytes from the
> -;   end of the firmware.
> +;   programmed to the RAM area defined by SEV_ES_AP_RESET_IP. The data
> +;   format is:
>  ;
> -;   0xffffffca (-0x36) - IP value
> -;   0xffffffcc (-0x34) - CS segment base [31:16]
> -;   0xffffffce (-0x32) - Size of the SEV-ES reset block
> -;   0xffffffd0 (-0x30) - SEV-ES reset block GUID
> -;                        (00f771de-1a7e-4fcb-890e-68c77e2fb44e)
> +;   IP value [0:15]
> +;   CS segment base [31:16]
> +;
> +;   GUID (SEV-ES reset block): 00f771de-1a7e-4fcb-890e-68c77e2fb44e
>  ;
>  ;   A hypervisor reads the CS segement base and IP value. The CS segment base
>  ;   value represents the high order 16-bits of the CS segment base, so the
> @@ -48,8 +67,6 @@ ALIGN   16
>  ;   program the EIP register with the IP value as read.
>  ;
>  
> -TIMES (32 - (sevEsResetBlockEnd - sevEsResetBlockStart)) DB 0
> -
>  sevEsResetBlockStart:
>      DD      SEV_ES_AP_RESET_IP
>      DW      sevEsResetBlockEnd - sevEsResetBlockStart
> @@ -57,6 +74,18 @@ sevEsResetBlockStart:
>      DB      0x89, 0x0E, 0x68, 0xC7, 0x7E, 0x2F, 0xB4, 0x4E
>  sevEsResetBlockEnd:
>  
> +;
> +; Table footer:
> +;
> +; length of whole table (16 bit word)
> +; GUID (table footer): 96b582de-1fb2-45f7-baea-a366c55a082d
> +;
> +    DW      guidedStructureEnd - guidedStructureStart
> +    DB      0xDE, 0x82, 0xB5, 0x96, 0xB2, 0x1F, 0xF7, 0x45
> +    DB      0xBA, 0xEA, 0xA3, 0x66, 0xC5, 0x5A, 0x08, 0x2D
> +
> +guidedStructureEnd:
> +
>  ALIGN   16
>  
>  applicationProcessorEntryPoint:
>