Re: [PATCH v2 2/3] x86/msi: don't use cached address and data fields in msi_desc for dump_msi()

[Jan Beulich <jbeulich@xxxxxxxx>]

On 06.03.2025 18:56, Roger Pau Monné wrote:
> On Thu, Mar 06, 2025 at 05:45:27PM +0100, Jan Beulich wrote:
>> On 06.03.2025 15:57, Roger Pau Monne wrote:
>>> Instead compose a dummy MSI message just for the purpose of getting the
>>> delivery attributes, which are the same for all messages.  Note that the
>>> previous usage of the cached MSI message wasn't fetching the hardware MSI
>>> fields either.
>>
>> This feels not future proof. There's no guarantee special IRQs (HPET, IOMMU)
>> would necessarily use msi_compose_msg() (or any open-coded subset thereof).
> 
> Hm, even if not using msi_compose_msg() I don't see how any device
> would use a different MSI settings from physical delivery and fixed
> destination.  I think it's unlikely for a device to use anything
> different from the current values set by msi_compose_msg().

I'm not entirely sure about this. If e.g. broadcast was possible to send via
MSI, I could see use cases. Potentially also for e.g. NMI.

> Otherwise I can see about returning whether the entry needs to be
> updated from iommu_update_ire_from_msi() (if the offset into the IRT
> for the entry has changed).  However that requires adding code to both
> AMD and Intel IOMMU implementations, and will need at least a way to
> signal that the MSI fields must forcefully be written on resume.

While that may indeed be more intrusive, it feels like it may be the less
risky approach (as to overlooking yet another case where we rely on the
untranslated message to be stored). Overall I can only say that I'm
uncertain which of the two approaches would end up being better.

>>> @@ -1512,8 +1520,7 @@ static void cf_check dump_msi(unsigned char key)
>>>              mask = '?';
>>>          printk(" %-6s%4u vec=%02x%7s%6s%3sassert%5s%7s"
>>>                 " dest=%08x mask=%d/%c%c/%c\n",
>>> -               type, irq,
>>> -               (data & MSI_DATA_VECTOR_MASK) >> MSI_DATA_VECTOR_SHIFT,
>>> +               type, irq, desc->arch.vector,
>>
>> We've already dropped desc's lock, so shouldn't be de-referencing desc 
>> anymore.
> 
> Right, I need to cache it before dropping the lock.
> 
>>>                 data & MSI_DATA_DELIVERY_LOWPRI ? "lowest" : "fixed",
>>>                 data & MSI_DATA_TRIGGER_LEVEL ? "level" : "edge",
>>>                 data & MSI_DATA_LEVEL_ASSERT ? "" : "de",
>>
>> To add to the comment at the top, plus taking patch 1 into account: If we
>> uniformly used the output of the dummy msi_compose_msg() invocation, why 
>> would
>> we even bother to log information conditionally upon what is in data/addr?
> 
> We could change what's set by msi_compose_msg(), and then the
> information here would be incorrect (if hardcoded).

Hmm, yes, that could happen, but would hopefully be caught in review (much
like it happened this time round).

Jan