Re: [PATCH v2 1/5] KVM: x86/xen: Restrict hypercall MSR to unofficial synthetic range

[Sean Christopherson]

On Sat, Feb 15, 2025, David Woodhouse wrote:
> On 15 February 2025 02:14:33 CET, Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
> >Reject userspace attempts to set the Xen hypercall page MSR to an index
> >outside of the "standard" virtualization range [0x40000000, 0x4fffffff],
> >as KVM is not equipped to handle collisions with real MSRs, e.g. KVM
> >doesn't update MSR interception, conflicts with VMCS/VMCB fields, special
> >case writes in KVM, etc.
> >
> >While the MSR index isn't strictly ABI, i.e. can theoretically float to
> >any value, in practice no known VMM sets the MSR index to anything other
> >than 0x40000000 or 0x40000200.

...

> This patch should probably have a docs update too.

To avoid sending an entirely new version only to discover I suck at writing docs,
how does this look?

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 2b52eb77e29c..5fe84f2427b5 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -1000,6 +1000,10 @@ blobs in userspace.  When the guest writes the MSR, kvm copies one
 page of a blob (32- or 64-bit, depending on the vcpu mode) to guest
 memory.
 
+The MSR index must be in the range [0x40000000, 0x4fffffff], i.e. must reside
+in the range that is unofficially reserved for use by hypervisors.  The min/max
+values are enumerated via KVM_XEN_MSR_MIN_INDEX and KVM_XEN_MSR_MAX_INDEX.
+
 ::
 
   struct kvm_xen_hvm_config {