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

[John Allen]

On Tue, Sep 16, 2025 at 05:55:33PM -0500, John Allen wrote:
> 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.

Ok, I think I see the problem. The cases above where we were seeing the
added print statements from kvm_set_msr_common were not situations where
we were going through the __kvm_emulate_msr_write via
sev_es_sync_from_ghcb. When we call __kvm_emulate_msr_write from this
context, we never end up getting to kvm_set_msr_common because we hit
the following statement at the top of svm_set_msr:

if (sev_es_prevent_msr_access(vcpu, msr))
	return vcpu->kvm->arch.has_protected_state ? -EINVAL : 0;

So I'm not sure if this would force using the original method of
directly setting arch.ia32_xss or if there's some additional handling
here that we need in this scenario to allow the msr access.

Thanks,
John