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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 14.05.2025 17:12, Roger Pau Monné wrote:
> 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(),

Right, this matches what I said ...

> 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).

... here.

Jan

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