Re: [PATCH v2 06/12] VT-d: respect ACPI SATC's ATC_REQUIRED flag

[Jan Beulich <jbeulich@xxxxxxxx>]

On 20.05.2024 13:36, Roger Pau Monné wrote:
> On Wed, May 15, 2024 at 12:42:40PM +0200, Jan Beulich wrote:
>> On 06.05.2024 15:38, Roger Pau Monné wrote:
>>> On Thu, Feb 15, 2024 at 11:16:11AM +0100, Jan Beulich wrote:
>>>> When the flag is set, permit Dom0 to control the device (no worse than
>>>> what we had before and in line with other "best effort" behavior we use
>>>> when it comes to Dom0),
>>>
>>> I think we should somehow be able to signal dom0 that this device
>>> might not operate as expected, otherwise dom0 might use it and the
>>> device could silently malfunction due to ATS not being enabled.
>>
>> Whatever signaling we invented, no Dom0 would be required to respect it,
>> and for (perhaps quite) some time no Dom0 kernel would even exist to query
>> that property.
>>
>>> Otherwise we should just hide the device from dom0.
>>
>> This would feel wrong to me, almost like a regression from what we had
>> before.
> 
> Exposing a device to dom0 that won't be functional doesn't seem like a
> very wise choice from Xen TBH.

Yes but. That's what we're doing right now, after all.

>>> I assume setting the IOMMU context entry to passthrough mode would
>>> also be fine for such devices that require ATS?
>>
>> I'm afraid I'm lacking the connection of the question to what is being
>> done here. Can you perhaps provide some more context? To provide some
>> context from my side: Using pass-through mode would be excluded when Dom0
>> is PVH. Hence why I'm not getting why we would want to even just consider
>> doing so.
>>
>> Yet, looking at the spec, in pass-through mode translation requests are
>> treated as UR. So maybe your question was towards there needing to be
>> handling (whichever way) for the case where pass-through mode was
>> requested for PV Dom0? The only half-way sensible thing to do in that case
>> that I can think of right now would be to ignore that command line option,
> 
> Hm, maybe I'm confused, but if the IOMMU device context entry is set
> in pass-through mode ATS won't be enabled and hence no translation
> requests would be send from the device?
> 
> IOW, devices listed in the SATC can only mandate ATS enabled when the
> IOMMU is enforcing translation.   IF the IOMMU is not enabled or if
> the device is in passthrough mode then the requirement for having ATS
> enabled no longer applies.

Oh, I think I now get what your original question was about: Instead of
enabling ATS on such devices, we might run them in pass-through mode.
For PV that would appear to be an option, yes. But with PVH (presumably)
being the future I'd be rather hesitant to go that route.

Jan