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[91.219.240.2]) by smtp.gmail.com with ESMTPSA id b145sm20013128wme.41.2020.05.14.11.14.31 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 14 May 2020 11:14:32 -0700 (PDT) From: Vitaly Kuznetsov To: Vivek Goyal Cc: kvm@vger.kernel.org, x86@kernel.org, Paolo Bonzini , Andy Lutomirski , Thomas Gleixner , Borislav Petkov , "H. Peter Anvin" , Wanpeng Li , Sean Christopherson , Jim Mattson , Gavin Shan , Peter Zijlstra , linux-kernel@vger.kernel.org Subject: Re: [PATCH 0/8] KVM: x86: Interrupt-based mechanism for async_pf 'page present' notifications In-Reply-To: <20200513141644.GD173965@redhat.com> References: <20200511164752.2158645-1-vkuznets@redhat.com> <20200513141644.GD173965@redhat.com> Date: Thu, 14 May 2020 20:14:31 +0200 Message-ID: <87lfluwfi0.fsf@vitty.brq.redhat.com> MIME-Version: 1.0 Content-Type: text/plain Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Vivek Goyal 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