Re: [PATCH] svm/avic: Allow avic_vcpu_load logic to support host APIC ID 255

From: Graf, Alexander
Date: Tue May 07 2019 - 10:17:20 EST

On 03.05.19 15:37, Suthikulpanit, Suravee wrote:
Current logic does not allow VCPU to be loaded onto CPU with
APIC ID 255. This should be allowed since the host physical APIC ID
field in the AVIC Physical APIC table entry is an 8-bit value,
and APIC ID 255 is valid in system with x2APIC enabled.

Instead, do not allow VCPU load if the host APIC ID cannot be
represented by an 8-bit value.

Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@xxxxxxx>

Your comment for AVIC_MAX_PHYSICAL_ID_COUNT says that 0xff (255) is broadcast hence you disallow that value. In fact, even the comment a few lines above the patch hunk does say that. Why the change of mind?


arch/x86/kvm/svm.c | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index 294448e..122788f 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -2071,7 +2071,11 @@ static void avic_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
if (!kvm_vcpu_apicv_active(vcpu))
- if (WARN_ON(h_physical_id >= AVIC_MAX_PHYSICAL_ID_COUNT))
+ /*
+ * Since the host physical APIC id is 8 bits,
+ * we can support host APIC ID upto 255.
+ */
+ if (WARN_ON(h_physical_id > AVIC_MAX_PHYSICAL_ID_COUNT))
entry = READ_ONCE(*(svm->avic_physical_id_cache));