KVM: x86/xen: Use 32-bit locked bts for vcpu_info evtchn_pending_sel

Replace test_and_set_bit() on vcpu_info->evtchn_pending_sel with an
explicit 'lock btsl' in kvm_xen_set_evtchn_fast(). The generic
test_and_set_bit() uses a 64-bit locked operation ('lock btsq') on
x86-64, and the address of the per-vCPU info is guest-controlled and
only required to be 32-bit aligned, so an 8-byte access can generate a
split-lock #AC exception.

Since evtchn_pending_sel is at most 64 bits wide and port_word_bit
ranges 0-63, a 32-bit 'lock btsl' suffices for both native and compat
vcpu_info layouts, and only requires the 4-byte alignment that is
already guaranteed by the registration path.

This also eliminates the bogus cast of compat_vcpu_info's 32-bit
evtchn_pending_sel to 'unsigned long *' which was the original source
of the split-lock hazard.

Note, KVM reuses the local gpc. The atomic accesses to pending_bits is
on the page-aligned per-VM shared_info structure, i.e. doesn't need the
same treatment as the access is guaranteed to be 64-bit aligned.

Fixes: 14243b387137 ("KVM: x86/xen: Add KVM_IRQ_ROUTING_XEN_EVTCHN and event channel delivery")
Reported-by: sashiko-bot@kernel.org
Closes: https://lore.kernel.org/all/20260604193554.1BA311F00893@smtp.kernel.org
Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Assisted-by: Claude:claude-mythos-5
Reviewed-by: Paul Durrant <paul@xen.org>
Link: https://patch.msgid.link/20260831213632.81023-8-dwmw2@infradead.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

0 files changed, 0 insertions, 0 deletions