KVM: SVM: Fix a benign off-by-one bug in AVIC physical table mask

commit a2f25d281cbe6bea4e0421beb8b52b5afd46e8e7 upstream.

Define the "physical table max index mask" as bits 8:0, not 9:0.  x2AVIC
currently supports a max of 512 entries, i.e. the max index is 511, and
the inputs to GENMASK_ULL() are inclusive.  The bug is benign as bit 9 is
reserved and never set by KVM, i.e. KVM is just clearing bits that are
guaranteed to be zero.

Note, as of this writing, APM "Rev. 3.39-October 2022" incorrectly states
that bits 11:8 are reserved in Table B-1. VMCB Layout, Control Area.  I.e.
that table wasn't updated when x2AVIC support was added.

Opportunistically fix the comment for the max AVIC ID to align with the
code, and clean up comment formatting too.

Fixes: eadd6be7bd64 ("KVM: SVM: Introduce logic to (de)activate x2AVIC mode")
Cc: stable@vger.kernel.org
Cc: Alejandro Jimenez <alejandro.j.jimenez@oracle.com>
Cc: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Tested-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Message-Id: <20230207002156.521736-2-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/arch/x86/include/asm/svm.h b/arch/x86/include/asm/svm.h
index 0361626841bc00da38c9dd1713e6995914d29e5c..236fda748ae9679de9053b98f87360ce5d180ec0 100644 (file)
--- a/arch/x86/include/asm/svm.h
+++ b/arch/x86/include/asm/svm.h
@@ -256,20 +256,22 @@ enum avic_ipi_failure_cause {
        AVIC_IPI_FAILURE_INVALID_BACKING_PAGE,
 };
 
-#define AVIC_PHYSICAL_MAX_INDEX_MASK   GENMASK_ULL(9, 0)
+#define AVIC_PHYSICAL_MAX_INDEX_MASK   GENMASK_ULL(8, 0)
 
 /*
- * For AVIC, the max index allowed for physical APIC ID
- * table is 0xff (255).
+ * For AVIC, the max index allowed for physical APIC ID table is 0xfe (254), as
+ * 0xff is a broadcast to all CPUs, i.e. can't be targeted individually.
  */
 #define AVIC_MAX_PHYSICAL_ID           0XFEULL
 
 /*
- * For x2AVIC, the max index allowed for physical APIC ID
- * table is 0x1ff (511).
+ * For x2AVIC, the max index allowed for physical APIC ID table is 0x1ff (511).
  */
 #define X2AVIC_MAX_PHYSICAL_ID         0x1FFUL
 
+static_assert((AVIC_MAX_PHYSICAL_ID & AVIC_PHYSICAL_MAX_INDEX_MASK) == AVIC_MAX_PHYSICAL_ID);
+static_assert((X2AVIC_MAX_PHYSICAL_ID & AVIC_PHYSICAL_MAX_INDEX_MASK) == X2AVIC_MAX_PHYSICAL_ID);
+
 #define AVIC_HPA_MASK  ~((0xFFFULL << 52) | 0xFFF)
 #define VMCB_AVIC_APIC_BAR_MASK                0xFFFFFFFFFF000ULL