linux-kernel - Re: [PATCH v2 3/4] KVM: async_pf: Force a nested vmexit if the injected #PF is async

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Date:   Fri, 16 Jun 2017 17:38:32 +0200
From:   Radim Krčmář <rkrcmar@...hat.com>
To:     Wanpeng Li <kernellwp@...il.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 v2 3/4] KVM: async_pf: Force a nested vmexit if the
 injected #PF is async_pf

2017-06-16 22:24+0800, Wanpeng Li:
> 2017-06-16 21:37 GMT+08:00 Radim Krčmář <rkrcmar@...hat.com>:
> > 2017-06-14 19:26-0700, Wanpeng Li:
> >> From: Wanpeng Li <wanpeng.li@...mail.com>
> >>
> >> Add an async_page_fault field to vcpu->arch.exception to identify an async
> >> page fault, and constructs the expected vm-exit information fields. Force
> >> a nested VM exit from nested_vmx_check_exception() if the injected #PF
> >> is async page fault.
> >>
> >> Cc: Paolo Bonzini <pbonzini@...hat.com>
> >> Cc: Radim Krčmář <rkrcmar@...hat.com>
> >> Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
> >> ---
> >> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> >> @@ -452,7 +452,11 @@ EXPORT_SYMBOL_GPL(kvm_complete_insn_gp);
> >>  void kvm_inject_page_fault(struct kvm_vcpu *vcpu, struct x86_exception *fault)
> >>  {
> >>       ++vcpu->stat.pf_guest;
> >> -     vcpu->arch.cr2 = fault->address;
> >> +     vcpu->arch.exception.async_page_fault = fault->async_page_fault;
> >
> > I think we need to act as if arch.exception.async_page_fault was not
> > pending in kvm_vcpu_ioctl_x86_get_vcpu_events().  Otherwise, if we
> > migrate with pending async_page_fault exception, we'd inject it as a
> > normal #PF, which could confuse/kill the nested guest.
> >
> > And kvm_vcpu_ioctl_x86_set_vcpu_events() should clean the flag for
> > sanity as well.
> 
> Do you mean we should add a field like async_page_fault to
> kvm_vcpu_events::exception, then saves arch.exception.async_page_fault
> to events->exception.async_page_fault through KVM_GET_VCPU_EVENTS and
> restores events->exception.async_page_fault to
> arch.exception.async_page_fault through KVM_SET_VCPU_EVENTS?

No, I thought we could get away with a disgusting hack of hiding the
exception from userspace, which would work for migration, but not if
local userspace did KVM_GET_VCPU_EVENTS and KVM_SET_VCPU_EVENTS ...

Extending the userspace interface would work, but I'd do it as a last
resort, after all conservative solutions have failed.
async_pf migration is very crude, so exposing the exception is just an
ugly workaround for the local case.  Adding the flag would also require
userspace configuration of async_pf features for the guest to keep
compatibility.

I see two options that might be simpler than adding the userspace flag:

 1) do the nested VM exit sooner, at the place where we now queue #PF,
 2) queue the #PF later, save the async_pf in some intermediate
    structure and consume it at the place where you proposed the nested
    VM exit.