[PATCH v3 2/3] x86/nestedsvm: honour L2's EFLAGS.IF when L1 clears V_INTR_MASKING

[chunjie.zhu@xxxxxxxxxx]

From: Chunjie Zhu <chunjie.zhu@xxxxxxxxxx>

L0 always runs L2 with V_INTR_MASKING set, so a physical interrupt exits to
L0 regardless of L2's EFLAGS.IF.  nsvm_intr_blocked() only blocks when L1
requested V_INTR_MASKING=1 and L1's IF was clear.  With V_INTR_MASKING=0
and L2's IF clear, nestedsvm_vcpu_interrupt() would still inject
VMEXIT(INTR) into L1.

Per the APM (vol.2 rev 3.44 "Physical Interrupt Masking in EFLAGS"), with
V_INTR_MASKING=0 L2's IF masks physical interrupts, and a masked interrupt
must stay pending.  Return NSVM_INTR_MASKED for external interrupts in
that case.

Signed-off-by: Chunjie Zhu <chunjie.zhu@xxxxxxxxxx>
---
v3:
 * Add ASSERT(v == current), as suggested by Ross.
 * Rebased on staging.

 xen/arch/x86/hvm/svm/nestedsvm.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/xen/arch/x86/hvm/svm/nestedsvm.c b/xen/arch/x86/hvm/svm/nestedsvm.c
index 40c7717c5572..706aed8ed026 100644
--- a/xen/arch/x86/hvm/svm/nestedsvm.c
+++ b/xen/arch/x86/hvm/svm/nestedsvm.c
@@ -1150,6 +1150,7 @@ enum hvm_intblk cf_check nsvm_intr_blocked(struct vcpu *v)
     struct nestedsvm *svm = &vcpu_nestedsvm(v);
     struct nestedvcpu *nv = &vcpu_nestedhvm(v);
 
+    ASSERT(v == current);
     ASSERT(nestedhvm_enabled(v->domain));
 
     if ( !nestedsvm_gif_isset(v) )
@@ -1159,7 +1160,16 @@ enum hvm_intblk cf_check nsvm_intr_blocked(struct vcpu 
*v)
     {
         struct vmcb_struct *n2vmcb = nv->nv_n2vmcx;
 
-        if ( svm->ns_hostflags.vintrmask && !svm->ns_hostflags.rflagsif )
+        /*
+         * With V_INTR_MASKING, L1's EFLAGS.IF masks physical interrupts.
+         * Without it, L2's EFLAGS.IF does (APM vol.2 rev 3.44 "Physical
+         * Interrupt Masking in EFLAGS"); L0 always runs L2 with
+         * V_INTR_MASKING set, so the interrupt would otherwise exit to L0
+         * and be forwarded to L1 even with L2's IF clear.
+         */
+        if ( svm->ns_hostflags.vintrmask
+             ? !svm->ns_hostflags.rflagsif
+             : !(guest_cpu_user_regs()->eflags & X86_EFLAGS_IF) )
             return hvm_intblk_rflags_ie;
 
         /* when l1 guest passes its devices through to the l2 guest
-- 
2.34.1