Re: [PATCH 1/5] KVM: x86/xen: Restrict hypercall MSR to unofficial synthetic range

[David Woodhouse]

On Wed, 2025-02-05 at 11:20 -0800, Sean Christopherson wrote:
> On Wed, Feb 05, 2025, David Woodhouse wrote:
> > On Wed, 2025-02-05 at 16:18 +0000, David Woodhouse wrote:
> > > 
> > > > Oh!  It doesn't help KVM avoid breaking userspace, but a way for QEMU to avoid a
> > > > future collision would be to have QEMU start at 0x40000200 when Hyper-V is enabled,
> > > > but then use KVM_GET_MSR_INDEX_LIST to detect a collision with KVM Hyper-V, e.g.
> > > > increment the index until an available index is found (with sanity checks and whatnot).
> > > 
> > > Makes sense. I think that's a third separate patch, yes?
> > 
> > To be clear, I think I mean a third patch which further restricts
> > kvm_xen_hvm_config() to disallow indices for which
> > kvm_is_advertised_msr() returns true?
> > 
> > We could roll that into your original patch instead, if you prefer.
> 
> Nah, I like the idea of separate patch.

Helpfully, kvm_is_advertised_msr() doesn't actually return true for
MSR_IA32_XSS. Is that a bug?

And kvm_vcpu_reset() attempts to set MSR_IA32_XSS even if the guest
doesn't have X86_FEATURE_XSAVES. Is that a bug?

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