Messages in this thread | | | Date | Thu, 1 Feb 2018 20:10:41 +0100 | From | Radim Krčmář <> | Subject | Re: [PATCH] KVM: x86: fix backward migration with async_PF |
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2018-02-01 13:09-0500, Paolo Bonzini: > On 01/02/2018 12:50, Radim Krčmář wrote: > > Guests on new hypersiors might set KVM_ASYNC_PF_DELIVERY_AS_PF_VMEXIT > > bit when enabling async_PF, but this bit is reserved on old hypervisors, > > which results in a failure upon migration. > > > > Guests at least expect that KVM_ASYNC_PF_DELIVERY_AS_PF_VMEXIT might not > > be present when booting, so we allow userspace to handle migration > > compatibility by adding a KVM CPUID flag that determines the presence of > > KVM_ASYNC_PF_DELIVERY_AS_PF_VMEXIT. > > > > Fixes: 52a5c155cf79 ("KVM: async_pf: Let guest support delivery of async_pf from guest mode") > > Cc: <stable@vger.kernel.org> > > Signed-off-by: Radim Krčmář <rkrcmar@redhat.com> > > This has to be documented in Documentation/virtual/kvm/cpuid.txt.
Will add, also to the MSR if we agree on v2.
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c > > index 4c3103f449a3..c16740a06f0c 100644 > > --- a/arch/x86/kvm/x86.c > > +++ b/arch/x86/kvm/x86.c > > @@ -2139,8 +2139,10 @@ static int kvm_pv_enable_async_pf(struct kvm_vcpu *vcpu, u64 data) > > { > > gpa_t gpa = data & ~0x3f; > > > > - /* Bits 3:5 are reserved, Should be zero */ > > - if (data & 0x38) > > + /* Bits 3:5 are reserved, Should be zero. */ > > + if (data & 0x38 || > > + (data & KVM_ASYNC_PF_DELIVERY_AS_PF_VMEXIT && > > + !guest_kvm_cpuid_has(vcpu, KVM_FEATURE_ASYNC_PF_VMEXIT))) > > return 1; > > > > vcpu->arch.apf.msr_val = data; > > > > This check will break migration if the source guest and host both have > the recent kernels which support KVM_ASYNC_PF_DELIVERY_AS_PF_VMEXIT, so > I am not sure about it. Otherwise, the patch is okay!
Good point, breaking forward migration is worse than doing nothing.
A compromise solution would be to drop the feature check from the hypervisor. Newer guests would work everywhere and there would be no change to old systems, so v4.13-v4.15 guests could at least upgrade.
Slightly better than doing nothing, IMO,
thanks.
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