Re: [PATCH v2 6/6] x86/HVM: limit cache writeback overhead

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Wed, May 14, 2025 at 03:20:56PM +0200, Jan Beulich wrote:
> On 14.05.2025 15:00, Roger Pau Monné wrote:
> > On Wed, May 03, 2023 at 11:47:18AM +0200, Jan Beulich wrote:
> >> There's no need to write back caches on all CPUs upon seeing a WBINVD
> >> exit; ones that a vCPU hasn't run on since the last writeback (or since
> >> it was started) can't hold data which may need writing back.
> > 
> > Couldn't you do the same with PV mode, and hence put the cpumask in
> > arch_vcpu rather than hvm_vcpu?
> 
> We could in principle, but there's no use of flush_all() there right now
> (i.e. nothing to "win").

Yes, that will get "fixed" if we take patch 2 from my series.  That
fixes the lack of broadcasting of wb{,no}invd when emulating it for
PV domains.

I think this patch would be better after my fix to cache_op(), and
then the uncertainty around patch 2 makes it unclear whether we want
this.

> >> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> >> ---
> >> With us not running AMD IOMMUs in non-coherent ways, I wonder whether
> >> svm_wbinvd_intercept() really needs to do anything (or whether it
> >> couldn't check iommu_snoop just like VMX does, knowing that as of
> >> c609108b2190 ["x86/shadow: make iommu_snoop usage consistent with
> >> HAP's"] that's always set; this would largely serve as grep fodder then,
> >> to make sure this code is updated once / when we do away with this
> >> global variable, and it would be the penultimate step to being able to
> >> fold SVM's and VT-x'es functions).
> > 
> > On my series I expand cache_flush_permitted() to also account for
> > iommu_snoop, I think it's easier to have a single check that signals
> > whether cache control is allowed for a domain, rather that having to
> > check multiple different conditions.
> 
> Right, adjustments here would want making (in whichever series goes in
> later).
> 
> For both of the responses: I think with patch 1 of this series having
> gone in and with in particular Andrew's concern over patch 2 (which
> may extend to patch 3), it may make sense for your series to go next.
> I shall then re-base, while considering what to do with patches 2 and 3
> (they may need dropping in the end).

Makes sense, I still need to get over your feedback on my series, I've
been distracted with other stuff.

Thanks, Roger.