On Tue, Mar 05, 2024, Tom Lendacky wrote: > On 3/4/24 11:55, Sean Christopherson wrote: > > +Tom > > > > "KVM: SVM:" for the shortlog scope. > > > > On Fri, Mar 01, 2024, Zheyun Shen wrote: > > > On AMD CPUs without ensuring cache consistency, each memory page reclamation in > > > an SEV guest triggers a call to wbinvd_on_all_cpus, thereby affecting the > > > performance of other programs on the host. > > > > > > Typically, an AMD server may have 128 cores or more, while the SEV guest might only > > > utilize 8 of these cores. Meanwhile, host can use qemu-affinity to bind these 8 vCPUs > > > to specific physical CPUs. > > > > > > Therefore, keeping a record of the physical core numbers each time a vCPU runs > > > can help avoid flushing the cache for all CPUs every time. > > > > This needs an unequivocal statement from AMD that flushing caches only on CPUs > > that do VMRUN is sufficient. That sounds like it should be obviously correct, > > as I don't see how else a cache line can be dirtied for the encrypted PA, but > > this entire non-coherent caches mess makes me more than a bit paranoid. > > As long as the wbinvd_on_all_cpus() related to the ASID flushing isn't > changed, this should be ok. And the code currently flushes the source pages > when doing LAUNCH_UPDATE commands and adding encrypted regions, so should be > good there. Nice, thanks! > Would it make sense to make this configurable, with the current behavior the > default, until testing looks good for a while? I don't hate the idea, but I'm inclined to hit the "I'm feeling lucky" button. I would rather we put in effort to all but guarantee we can do a clean revert in the future, at which point a kill switch doesn't add all that much value. E.g. it would allow for a non-disruptive fix, and maybe a slightly faster confirmation of a bug, but that's about it. And since the fallout from this would be host data corruption, _not_ rebooting hosts that may have been corrupted is probably a bad idea, i.e. the whole non-disruptive fix benefit is quite dubious. The other issue is that it'd be extremely difficult to know when we could/should remove the kill switch. It might be months or even years before anyone starts running high volume of SEV/SEV-ES VMs with this optimization.
