Re: [PATCH] hw/i386/pc: Document pc_system_ovmf_table_find

[Dov Murik <dovmurik@linux.ibm.com>]

On 29/06/2021 1:03, Tom Lendacky wrote:
> On 6/22/21 7:58 AM, Dov Murik wrote:
>> +cc: Tom Lendacky
>>
>> On 22/06/2021 15:47, Philippe Mathieu-Daudé wrote:
>>> On 6/22/21 2:44 PM, Dov Murik wrote:
>>>> Suggested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
>>>> Signed-off-by: Dov Murik <dovmurik@linux.ibm.com>
>>>> ---
>>>>  hw/i386/pc_sysfw.c | 14 ++++++++++++++
>>>>  1 file changed, 14 insertions(+)
>>>>
>>>> diff --git a/hw/i386/pc_sysfw.c b/hw/i386/pc_sysfw.c
>>>> index 6ce37a2b05..e8d20cb83f 100644
>>>> --- a/hw/i386/pc_sysfw.c
>>>> +++ b/hw/i386/pc_sysfw.c
>>>> @@ -176,6 +176,20 @@ static void pc_system_parse_ovmf_flash(uint8_t *flash_ptr, size_t flash_size)
>>>>      ovmf_table += tot_len;
>>>>  }
>>>>  
>>>> +/**
>>>> + * pc_system_ovmf_table_find - Find the data associated with an entry in OVMF's
>>>> + * reset vector GUIDed table.
>>>> + *
>>>> + * @entry: GUID string of the entry to lookup
>>>> + * @data: Filled with a pointer to the entry's value (if not NULL)
>>>> + * @data_len: Filled with the length of the entry's value (if not NULL). Pass
>>>> + *            NULL here if the length of data is known.
>>>> + *
>>>> + * Note that this function must be called after the OVMF table was found and
>>>> + * copied by pc_system_parse_ovmf_flash().
>>>
>>> What about replacing this comment by:
>>>
>>>   assert(ovmf_table && ovmf_table_len);
>>>
>>
>> I think this will break things: in target/i386/sev.c we have SEV-ES code
>> that calls pc_system_ovmf_table_find() and can deal with the case when
>> there's no OVMF table.  An assert will break it.
> 
> Right, what would be best is to differentiate between an OVMF table that
> isn't present in the flash vs the fact that pc_system_parse_ovmf_flash()
> wasn't called, asserting only on the latter.
> 

[+cc James who wrote this code]


Thanks Tom; I agree.  To achieve that, we need one of these:

(a) add a 'static bool ovmf_table_parsed' which will be set to true at
the beginning of pc_system_parse_ovmf_flash(). Then, at the beginning of
pc_system_ovmf_table_find add: assert(ovmf_table_parsed).

(b) (ab)use our existing ovmf_table_len static variable: initialize it
to -1 (meaning that we haven't parsed the OVMF flash yet). When looking
for the table set it to 0 (meaning that OVMF table doesn't exist or is
invalid). When a proper table is found and copied to ovmf_table, then
set it to the real length (>= 0). At the beginning of
pc_system_ovmf_table_find add: assert(ovmf_table_len != -1). (this -1
can be #define OVMF_FLASH_NOT_PARSED -1).


Phil, Tom, James: which do you prefer? other options? Rust enum? ;-)


Thanks,
Dov


> Thanks,
> Tom
> 
>>
>>
>>> Otherwise,
>>>
>>> Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
>>>
>>
>> Thanks!
>>
>> -Dov
>>
>>
>>
>>> Thanks!
>>>
>>>> + *
>>>> + * Return: true if the entry was found in the OVMF table; false otherwise.
>>>> + */
>>>>  bool pc_system_ovmf_table_find(const char *entry, uint8_t **data,
>>>>                                 int *data_len)
>>>>  {
>>>>
>>>