Re: [edk2-devel] [PATCH 1/1] OvmfPkg/ResetVector: send post codes to qemu debug console

["Lendacky, Thomas via groups.io" <thomas.lendacky=amd.com@groups.io>]

On 1/24/24 13:24, Tom Lendacky wrote:
> On 1/24/24 10:47, Laszlo Ersek wrote:
>> On 1/24/24 16:31, Gerd Hoffmann wrote:
>>> Neat when doing ResetVector coding.
>>>
>>> Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
>>> ---
>>>   OvmfPkg/ResetVector/DebugCon.asm      | 43 +++++++++++++++++++++++++++
>>>   OvmfPkg/ResetVector/ResetVector.nasmb |  2 +-
>>>   2 files changed, 44 insertions(+), 1 deletion(-)
>>>   create mode 100644 OvmfPkg/ResetVector/DebugCon.asm
>>>
>>> diff --git a/OvmfPkg/ResetVector/DebugCon.asm 
>>> b/OvmfPkg/ResetVector/DebugCon.asm
>>> new file mode 100644
>>> index 000000000000..9c57d1a52c75
>>> --- /dev/null
>>> +++ b/OvmfPkg/ResetVector/DebugCon.asm
>>> @@ -0,0 +1,43 @@
>>> +;------------------------------------------------------------------------------
>>> +; @file
>>> +; qemu debug console support macros (based on serial port macros)
>>> +;
>>> +; Copyright (c) 2008 - 2018, Intel Corporation. All rights reserved.<BR>
>>> +; SPDX-License-Identifier: BSD-2-Clause-Patent
>>> +;
>>> +;------------------------------------------------------------------------------
>>> +
>>> +%macro  outToDebugPort 1
>>> +    mov     dx, 0x402
>>> +    mov     al, %1
>>> +    out     dx, al
>>> +%endmacro
>>> +
>>> +%macro  debugShowCharacter 1
>>> +    outToDebugPort %1
>>> +%endmacro
>>> +
>>> +%macro  debugShowHexDigit 1
>>> +  %if (%1 < 0xa)
>>> +    debugShowCharacter BYTE ('0' + (%1))
>>> +  %else
>>> +    debugShowCharacter BYTE ('a' + ((%1) - 0xa))
>>> +  %endif
>>> +%endmacro
>>> +
>>> +%macro  debugNewline 0
>>> +    debugShowCharacter `\r`
>>> +    debugShowCharacter `\n`
>>> +%endmacro
>>> +
>>> +%macro  debugShowPostCode 1
>>> +    debugShowHexDigit (((%1) >> 4) & 0xf)
>>> +    debugShowHexDigit ((%1) & 0xf)
>>> +    debugNewline
>>> +%endmacro
>>> +
>>> +BITS    16
>>> +
>>> +%macro  debugInitialize 0
>>> +    ; not required
>>> +%endmacro
>>> diff --git a/OvmfPkg/ResetVector/ResetVector.nasmb 
>>> b/OvmfPkg/ResetVector/ResetVector.nasmb
>>> index 5832aaa8abf7..f1655ddfcde3 100644
>>> --- a/OvmfPkg/ResetVector/ResetVector.nasmb
>>> +++ b/OvmfPkg/ResetVector/ResetVector.nasmb
>>> @@ -41,7 +41,7 @@
>>>   %elifdef DEBUG_SERIAL
>>>     %include "SerialDebug.asm"
>>>   %else
>>> -  %include "DebugDisabled.asm"
>>> +  %include "DebugCon.asm"
>>>   %endif
>>>   %include "Ia32/SearchForBfvBase.asm"
>>
>> (1) How much output does this produce? Trapping to QEMU for every single
>> character written to the debug console is very slow. If it only produces
>> a few bytes, that should be OK.
>>
>> (2) debugInitialize could actually be put to use; the presence of the
>> QEMU debug console is detectable. We'd need to allocate a single byte
>> somewhere, detect the console in debugInitialize, save the result in
>> that byte, then check the byte in debugShowPostCode. Eliminates even
>> those few (?) traps if there is no debug console configured.
>>
>> (3) I'm already hating myself for asking about this, but... is there a
>> way for only customizing debugInitialize and debugShowCharacter? The
>> rest is common with "SerialDebug.asm", and I wish I hadn't had to review
>> those (unchanged) macros, they make my eyes bleed :/
>>
>> Anyway, if you think any of these (or all of these!) means too much
>> work, I'm OK with the patch going-in as is.
> 
> My concern would be around SEV-ES/SEV-SNP guest usage. The in and out 
> instruction will generate a #VC and would require a #VC handler to be in 
> place.
> 
> I'm pretty sure this will cause issues for those types of guests, but it 
> might be a few days before I can get to it to verify. @Gerd, were you able 
> to test this with those types of guests?

Had a meeting get canceled and so got a chance to test this. As I thought, 
this causes SEV-ES/SEV-SNP guest failures.

Thanks,
Tom

> 
> Thanks,
> Tom
> 
>>
>> I'm a bit concerned about (1), TBH.
>>
>> Laszlo
>>


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