On Fri, May 28, 2021, Lai Jiangshan wrote: > > > On 2021/5/28 03:28, Sean Christopherson wrote: > > On Thu, May 27, 2021, Sean Christopherson wrote: > > > > KVM_REQ_MMU_RELOAD is overkill, nuking the shadow page tables will completely > > > > offset the performance gains of the paravirtualized flush. > > > > Argh, I take that back. The PV KVM_VCPU_FLUSH_TLB flag doesn't distinguish > > between flushing a specific mm and flushing the entire TLB. The HyperV usage > > (via KVM_REQ) also throws everything into a single bucket. A full RELOAD still > > isn't necessary as KVM just needs to sync all roots, not blast them away. For > > previous roots, KVM doesn't have a mechanism to defer the sync, so the immediate > > fix will need to unload those roots. > > > > And looking at KVM's other flows, __kvm_mmu_new_pgd() and kvm_set_cr3() are also > > broken with respect to previous roots. E.g. if the guest does a MOV CR3 that > > flushes the entire TLB, followed by a MOV CR3 with PCID_NOFLUSH=1, KVM will fail > > to sync the MMU on the second flush even though the guest can technically rely > > on the first MOV CR3 to have synchronized any previous changes relative to the > > fisrt MOV CR3. > > Could you elaborate the problem please? > When can a MOV CR3 that needs to flush the entire TLB if PCID is enabled? Scratch that, I was wrong. The SDM explicitly states that other PCIDs don't need to be flushed if CR4.PCIDE=1.
