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

["Roger Pau Monné" <roger.pau@citrix.com>]

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.

> > 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.

Thanks, Roger.