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

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

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?

> Signed-off-by: Jan Beulich <jbeulich@suse.com>
> ---
> 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.

Thanks, Roger.