Re: [PATCH] KVM: x86/xen: Add KVM_XEN_VCPU_ATTR_TYPE_WRITE_HYPERCALL_PAGE

[Paul Durrant <xadimgnik@gmail.com>]

On 28/04/2026 21:12, David Woodhouse wrote:
> From: David Woodhouse <dwmw@amazon.co.uk>
> 
> Commit 3617c0ee7decb ("KVM: x86/xen: Only write Xen hypercall page for
> guest writes to MSR") blocked host-initiated writes from triggering the
> Xen hypercall page setup, to fix an SRCU usage violation when the
> hypercall MSR index collides with a real MSR written during vCPU reset.
> 
> However, some VMMs legitimately need to trigger hypercall page setup
> from host context. For example, a VMM may intercept the guest's MSR
> write to track an epoch (for kexec/crash recovery), and then replay the
> write as a host-initiated KVM_SET_MSRS to populate the hypercall page.
> The host_initiated check breaks this use case.
> 
> Add KVM_XEN_VCPU_ATTR_TYPE_WRITE_HYPERCALL_PAGE as a new vcpu attribute
> that explicitly invokes kvm_xen_write_hypercall_page() under proper
> locking. This gives userspace a safe interface to trigger hypercall page
> setup without going through the MSR write path, preserving the
> host_initiated defence in depth while restoring the lost functionality.
> 
> Fixes: 3617c0ee7dec ("KVM: x86/xen: Only write Xen hypercall page for guest writes to MSR")
> Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
> ---
>   Documentation/virt/kvm/api.rst                | 11 +++
>   arch/x86/include/uapi/asm/kvm.h               |  3 +
>   arch/x86/kvm/x86.c                            |  3 +-
>   arch/x86/kvm/xen.c                            |  7 ++
>   .../selftests/kvm/x86/xen_vmcall_test.c       | 96 +++++++++++++++++++
>   5 files changed, 119 insertions(+), 1 deletion(-)
> 
Reviewed-by: Paul Durrant <paul@xen.org>