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


Laszlo Ersek
 

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

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