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@xxxxxxxxxxxxxxx>
> > Signed-off-by: Radim Krčmář <rkrcmar@xxxxxxxxxx>
>
> 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.