Messages in this thread | | | From | Vitaly Kuznetsov <> | Subject | Re: [PATCH 0/8] KVM: x86: Interrupt-based mechanism for async_pf 'page present' notifications | Date | Thu, 14 May 2020 20:14:31 +0200 |
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Vivek Goyal <vgoyal@redhat.com> writes:
> On Mon, May 11, 2020 at 06:47:44PM +0200, Vitaly Kuznetsov wrote: >> Concerns were expressed around (ab)using #PF for KVM's async_pf mechanism, >> it seems that re-using #PF exception for a PV mechanism wasn't a great >> idea after all. The Grand Plan is to switch to using e.g. #VE for 'page >> not present' events and normal APIC interrupts for 'page ready' events. >> This series does the later. > > Hi Vitaly, > > How does any of this impact nested virtualization code (if any). > > I have tried understanding that logic, but I have to admit, I could > never get it. > > arch/x86/kvm/mmu/mmu.c > > int kvm_handle_page_fault(struct kvm_vcpu *vcpu, u64 error_code, > u64 fault_address, char *insn, int insn_len) > { > switch (vcpu->arch.apf.host_apf_reason) { > case KVM_PV_REASON_PAGE_NOT_PRESENT: > kvm_async_pf_task_wait(fault_address, 0); > case KVM_PV_REASON_PAGE_READY: > kvm_async_pf_task_wake(fault_address); > } > } >
"[PATCH 8/8] KVM: x86: drop KVM_PV_REASON_PAGE_READY case from kvm_handle_page_fault()" modifies this a little bit.
Basically (and if I understand this correctly) we have the following APF related feature (bit 2 in MSR_KVM_ASYNC_PF_EN): "asynchronous page faults are delivered to L1 as #PF vmexits.". When enabled, it allows L0 to inject #PF when L2 guest is running. L1 will see this as '#PF vmexit' and the code you cite will do exactly what do_async_page_fault() is doing.
When we switch to interrupt based delivery for 'page ready' events we don't need a special handling for them in L1 (as we don't need any special handling for all interrupts from devices in kernel when KVM guest is running).
I have to admit I haven't tested nested scenario yet, "what could go wrong?" :-)
-- Vitaly
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