[PATCH] kvm: document lapic regs field

["Michael S. Tsirkin" <mst@redhat.com>]

The logic in find_highest_vector looks
strange until you realize the reason for the
weird memory layout, which is because this is
what the CPU microcode expects.

Add a comment so this stops tripping people up.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
 arch/x86/kvm/lapic.h |    5 +++++
 1 files changed, 5 insertions(+), 0 deletions(-)

diff --git a/arch/x86/kvm/lapic.h b/arch/x86/kvm/lapic.h
index 41c62c7..1d1ec08 100644
--- a/arch/x86/kvm/lapic.h
+++ b/arch/x86/kvm/lapic.h
@@ -15,6 +15,11 @@ struct kvm_lapic {
 	bool irr_pending;
 	s16 isr_count;
 	int isr_cache;
+	/**
+	 * APIC register page.  The layout matches the register layout seen by
+	 * the guest 1:1, because it is accessed by the vmx microcode.
+	 * Note: Only one register, the TPR, is used by the microcode.
+	 */
 	void *regs;
 	gpa_t vapic_addr;
 	struct page *vapic_page;
-- 
MST