Re: [PATCH v2 3/6] OvmfPkg: convert ES Reset Block structure to be guided

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

On 11/20/20 19:45, 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
> ---
>  OvmfPkg/ResetVector/Ia16/ResetVectorVtf0.asm | 49 +++++++++++++++-----
>  1 file changed, 38 insertions(+), 11 deletions(-)

(1) Please update the subject line to:

OvmfPkg/ResetVector: convert SEV-ES Reset Block structure to be GUIDed

- edk2 prefers including module names too in the patch subjects
- "ES" is harder to understand than "SEV-ES"
- "GUIDed" is harder to misread as "guided"
- subject length is still OK (70 chars)


>
> diff --git a/OvmfPkg/ResetVector/Ia16/ResetVectorVtf0.asm b/OvmfPkg/ResetVector/Ia16/ResetVectorVtf0.asm
> index 980e0138e7fe..baf9d09f3625 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 (32 - ((guidedStructureEnd - guidedStructureStart) % 32)) DB 0

(2) This will insert 32 zero bytes if the size is already aligned to 32
bytes (because 32-0 = 32). In other words, the above produces 1 to 32
zero bytes, dependent on table size.

The variant I proposed in point (5) at

  https://edk2.groups.io/g/devel/message/67621
  https://www.redhat.com/archives/edk2-devel-archive/2020-November/msg00781.html

takes this into account, and only prepends 0 to 31 bytes (inclusive):

  TIMES (31 - (guidedStructureEnd - guidedStructureStart + 31) % 32) DB 0

- This variant subtracts 1 inside the remainder operation (which is
  expressed as adding 31).

- For compensation, it adds 1 just outside of the remainder operation.
  This addition in effect increases the subtrahend for the leftmost 32.
  Therefore this (-1) addend is ultimately folded into the leftmost 32,
  yielding 31 on the leftmost side.

  TIMES (32 - ((guidedStructureEnd - guidedStructureStart - 1) % 32 + 1)) DB 0
                                                          ^^^       ^^^
                                                          |         |
                                                          |         compensate
                                                          |         in the
                                                          |         remainder
                                                          |
                                                          slide down residue
                                                          class modulo 32


 TIMES (32 - ((guidedStructureEnd - guidedStructureStart + 31) % 32) - 1) DB 0
                                                         ^^^^        ^^^
                                                         |           |
                                                         |           unnest
                                                         |           increment
                                                         |           from
                                                         |           subtrahend
                                                         |
                                                         express modular
                                                         subtraction as
                                                         addition, to avoid
                                                         using % on a negative
                                                         integer (in case size
                                                         were 0)

 TIMES (31 - ((guidedStructureEnd - guidedStructureStart + 31) % 32)) DB 0
        ^^
        |
        fold previous (-1) addend into leftmost constant

- This juggling of 1 results in no changes for residue classes 1 through
  31, but wraps the outermost result (the padding size) for residue
  class 0, from 32 to 0.


> +
> +; Guided structure.  To traverse this you should first verify the
> +; presence of the table header guid

(3) suggest "table footer GUID" (the GUID follows the data, in address
order)

> +; (96b582de-1fb2-45f7-baea-a366c55a082d) at 0xffffffd0.  If that
> +; is found, the two bytes at 0xffffffce are the entire table length.

(4) can we make the whole table size field 32-bit? I don't have a
particular use case in mind, it just looks more extensible than 16-bit.
We can still keep the individual structs we have in mind 16-bit sized.

> +;
> +; The table is composed of structures with the form:
> +;
> +; Data (arbitrary bytes identified by guid)
> +; length from start of guid to end of data (2 bytes)

(5) This is hard to interpret, as "data" precedes "guid" in address
space (guid is a footer, not a header).

I suggest "length from start of data to end of GUID"


> +; guid (16 bytes)
> +;
> +; so work back from the header using the length to traverse until you

(6) suggest "from the footer"


> +; either find the guid you're looking for or run off the end of the
> +; table.

(7) suggest "run off the beginning of the table"

... I realize "start" and "end" can be interpreted temporally and
spatially. In a forward traversal they are the same, but now they
aren't. I suggest we use the spatial (address space order)
interpretation.

> +;
> +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]
> +;
> +;   SEV-ES reset block GUID: 00f771de-1a7e-4fcb-890e-68c77e2fb44e

(8) Did I understand from the v1 discussion that the corresponding QEMU
parser is not upstream yet? (Or at least not released?)

(9) The 16-bit size field of the SEV-ES reset block structure is not
documented.


>  ;
>  ;   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 +74,16 @@ sevEsResetBlockStart:
>      DB      0x89, 0x0E, 0x68, 0xC7, 0x7E, 0x2F, 0xB4, 0x4E
>  sevEsResetBlockEnd:
>
> +;
> +; Table header: length of whole table followed by table header

(10) I suggest "table footer" (twice)


> +; guid: 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:
>

Thanks!
Laszlo