[PATCH] i386: Don't try to set MSR_KVM_ASYNC_PF_EN if kernel-irqchip=off

[Eduardo Habkost <ehabkost@redhat.com>]

This addresses the following crash when running Linux v5.8
with kernel-irqchip=off:

  qemu-system-x86_64: error: failed to set MSR 0x4b564d02 to 0x0
  qemu-system-x86_64: ../target/i386/kvm.c:2714: kvm_buf_set_msrs: Assertion `ret == cpu->kvm_msr_buf->nmsrs' failed.

There is a kernel-side fix for the issue too (kernel commit
d831de177217 "KVM: x86: always allow writing '0' to
MSR_KVM_ASYNC_PF_EN"), but it's nice to simply not trigger
the bug if running an older kernel.

Fixes: https://bugs.launchpad.net/bugs/1896263
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
---
 target/i386/kvm.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/target/i386/kvm.c b/target/i386/kvm.c
index 9efb07e7c83..1492f41349f 100644
--- a/target/i386/kvm.c
+++ b/target/i386/kvm.c
@@ -2818,7 +2818,12 @@ static int kvm_put_msrs(X86CPU *cpu, int level)
         kvm_msr_entry_add(cpu, MSR_IA32_TSC, env->tsc);
         kvm_msr_entry_add(cpu, MSR_KVM_SYSTEM_TIME, env->system_time_msr);
         kvm_msr_entry_add(cpu, MSR_KVM_WALL_CLOCK, env->wall_clock_msr);
-        if (env->features[FEAT_KVM] & (1 << KVM_FEATURE_ASYNC_PF)) {
+        /*
+         * Some kernel versions (v5.8) won't let MSR_KVM_ASYNC_PF_EN to be set
+         * at all if kernel-irqchip=off, so don't try to set it in that case.
+         */
+        if (env->features[FEAT_KVM] & (1 << KVM_FEATURE_ASYNC_PF) &&
+            kvm_irqchip_in_kernel()) {
             kvm_msr_entry_add(cpu, MSR_KVM_ASYNC_PF_EN, env->async_pf_en_msr);
         }
         if (env->features[FEAT_KVM] & (1 << KVM_FEATURE_PV_EOI)) {
-- 
2.26.2