Re: [PATCH v15 29/41] KVM: SEV: Synchronize MSR_IA32_XSS from the GHCB when it's valid

John Allen
2025-09-16T22:55:46Z

On Tue, Sep 16, 2025 at 02:38:52PM -0700, Sean Christopherson wrote:
> On Tue, Sep 16, 2025, John Allen wrote:
> > On Tue, Sep 16, 2025 at 12:53:58PM -0700, Sean Christopherson wrote:
> > > On Tue, Sep 16, 2025, John Allen wrote:
> > > > On Fri, Sep 12, 2025 at 04:23:07PM -0700, Sean Christopherson wrote:
> > > > > diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> > > > > index 0cd77a87dd84..0cd32df7b9b6 100644
> > > > > --- a/arch/x86/kvm/svm/sev.c
> > > > > +++ b/arch/x86/kvm/svm/sev.c
> > > > > @@ -3306,6 +3306,9 @@ static void sev_es_sync_from_ghcb(struct vcpu_svm *svm)
> > > > > if (kvm_ghcb_xcr0_is_valid(svm))
> > > > > __kvm_set_xcr(vcpu, 0, kvm_ghcb_get_xcr0(ghcb));
> > > > >
> > > > > + if (kvm_ghcb_xss_is_valid(svm))
> > > > > + __kvm_emulate_msr_write(vcpu, MSR_IA32_XSS, kvm_ghcb_get_xss(ghcb));
> > > > > +
> > > >
> > > > It looks like this is the change that caused the selftest regression
> > > > with sev-es. It's not yet clear to me what the problem is though.
> > >
> > > Do you see any WARNs in the guest kernel log?
> > >
> > > The most obvious potential bug is that KVM is missing a CPUID update, e.g. due
> > > to dropping an XSS write, consuming stale data, not setting cpuid_dynamic_bits_dirty,
> > > etc. But AFAICT, CPUID.0xD.1.EBX (only thing that consumes the current XSS) is
> > > only used by init_xstate_size(), and I would expect the guest kernel's sanity
> > > checks in paranoid_xstate_size_valid() to yell if KVM botches CPUID emulation.
> >
> > Yes, actually that looks to be the case:
> >
> > [ 0.463504] ------------[ cut here ]------------
> > [ 0.464443] XSAVE consistency problem: size 880 != kernel_size 840
> > [ 0.465445] WARNING: CPU: 0 PID: 0 at arch/x86/kernel/fpu/xstate.c:638 paranoid_xstate_size_valid+0x101/0x140
>
> Can you run with the below printk tracing in the host (and optionally tracing in
> the guest for its updates)? Compile tested only.

Interesting, I see "Guest CPUID doesn't have XSAVES" times the number of
cpus followed by "XSS already set to val = 0, eliding updates" times the
number of cpus. This is with host tracing only. I can try with guest
tracing too in the morning.

Thanks,
John