Re: [PATCH 1/3] KVM: SVM: Check EFER.SVME before updating MSR_VM_CR on guest writes

[Sean Christopherson]

On Thu, Oct 29, 2026, Jim Mattson wrote:
> On Thu, Oct 29, 2026 at 2:19 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
> >
> > On Thu, Oct 29, 2026, Jim Mattson wrote:
> > > (Host- initiated writes are allowed to set any valid MSR_VM_CR bits.)
> >
> > Eww.  Why?  If we allow this, then we'll end up with the mess where the guest
> > can't write back the current value (if userspace is being weird).  I get that
> > we want to avoid ordering issues, but there's *already* a required order here,
> > unless userspace ignores the failure from svm_set_vm_cr() and continues on.
> >
> > KVM doesn't advertise the MSR as something to be saved/restored, and none of
> > QEMU, CHV, CrosVM, or our VMM appear to save/restore it.  So why make trouble
> > for ourselves?
> 
> OMG! I didn't look any farther than the obviously broken svm_set_vm_cr().
> 
> It looks like svm->nested.vm_cr_msr controls nothing other than itself.

LOL, nice.

> It's just 5 bits of storage with some weird locking rules, and it
> resets to 0 on suspend/resume.
> 
> What is the point of such half-baked support?

More than likely, so that older kernels didn't crash when writing an MSR KVM
didn't emulate. :-(