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Date: Thu, 15 Jun 2017 10:43:59 +0800 From: Wanpeng Li <kernellwp@...il.com> To: Radim Krčmář <rkrcmar@...hat.com> Cc: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>, Paolo Bonzini <pbonzini@...hat.com>, Wanpeng Li <wanpeng.li@...mail.com> Subject: Re: [PATCH 3/4] KVM: async_pf: Force a nested vmexit if the injected #PF is async_pf 2017-06-15 0:18 GMT+08:00 Radim Krčmář <rkrcmar@...hat.com>: > 2017-06-14 22:32+0800, Wanpeng Li: >> 2017-06-14 21:20 GMT+08:00 Radim Krčmář <rkrcmar@...hat.com>: >> > 2017-06-14 21:02+0800, Wanpeng Li: >> >> 2017-06-14 20:52 GMT+08:00 Radim Krčmář <rkrcmar@...hat.com>: >> >> > 2017-06-14 09:07+0800, Wanpeng Li: >> >> >> 2017-06-14 2:55 GMT+08:00 Radim Krčmář <rkrcmar@...hat.com>: >> >> >> > Using vcpu->arch.cr2 is suspicious as VMX doesn't update CR2 on VM >> >> >> > exits; isn't this going to change the CR2 visible in L2 guest after a >> >> >> > nested VM entry? >> >> >> >> >> >> Sorry, I don't fully understand the question. As you know this >> >> >> vcpu->arch.cr2 which includes token is set before async pf injection, >> >> > >> >> > Yes, I'm thinking that setting vcpu->arch.cr2 is a mistake in this case. >> >> > >> >> >> and L1 will intercept it from EXIT_QUALIFICATION during nested vmexit, >> >> > >> >> > Right, so we do not need to have the token in CR2, because L1 is not >> >> > going to look at it. >> >> > >> >> >> why it can change the CR2 visible in L2 guest after a nested VM entry? >> >> > >> >> > Sorry, the situation is too convoluted to be expressed in one sentence: >> >> > >> >> > 1) L2 is running with CR2 = L2CR2 >> >> > 3) VMX exits (say, unrelated EXTERNAL_INTERRUPT) and L0 stores L2CR2 in >> >> > vcpu->arch.cr2 >> >> > 2) APF for L1 has completed >> >> > 4) L0 KVM wants to inject APF and sets vcpu->arch.cr2 = APFT >> >> > 5) L0 KVM does a nested VM exit to L1, EXIT_QUALIFICATION = APFT >> >> > 6) L0 KVM enters L1 with CR2 = vcpu->arch.cr2 = APFT >> >> > 7) L1 stores APFT as L2's CR2 >> >> > 8) L1 handles APF, maybe reschedules, but eventually comes back to this >> >> > L2's thread >> >> > 9) after some time, L1 enters L2 with CR2 = APFT >> >> > 10) L2 is running with CR2 = APTF >> >> > >> >> > The original L2CR2 is lost and we'd introduce a bug if L2 wanted to look >> >> > at it, e.g. it was in a process of handling its #PF. >> >> >> >> Good point. What's your proposal? :) >> > >> > Get rid of async_pf. :) Optimal solutions aside, I think it would be >> > best to add a new injection function for APF. One that injects a normal >> > #PF for non-nested guests and directly triggers a #PF VM exit otherwise, >> > and call it from kvm_arch_async_page_*present(). >> >> In addition, nested vmexit in kvm_arch_async_page_*present() directly >> instead of through inject_pending_event() before vmentry, or nested >> vmexit after vmexit on L0 looks strange. > > Right, it might be tricky if another exception can get queued in > between. (Which shouldn't happen, though, because async_pf exceptions > must not cause double faults for no good reason.) > >> So how about the proposal of >> the nested_apf_token stuff? Radim, Paolo? > > I think it is worth exploring. We need to make sure that interfacing > with userspace through kvm_vcpu_ioctl_x86_{set,get}_vcpu_events() is > right, but it should be possible without any extension as migration is > already covered by unconditional async_pf wakeup on the destination. > > At this point, using a structure other than arch.exception would make > sense too -- async_pf uses the exception injection path mostly for > convenience, but the paravirt exception does not want to mix with real > exceptions. Yeah, but maybe need more reconstruct, I just send out v2 to fix it simply and avoid too aggressive. :) Regards, Wanpeng Li
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