Eduardo Habkost <ehabkost@xxxxxxxxxx> writes: > 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@xxxxxxxxxx> > --- > 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)) { I'm not sure kvm_irqchip_in_kernel() was required before we switched to interrupt-based APF (as we were always injecting #PF) but with kernel-5.8+ this should work. We'll need to merge this with https://lists.nongnu.org/archive/html/qemu-devel/2020-09/msg02963.html (queued by Paolo) and https://lists.nongnu.org/archive/html/qemu-devel/2020-09/msg06196.html which fixes a bug in it. as kvm_irqchip_in_kernel() should go around both KVM_FEATURE_ASYNC_PF and KVM_FEATURE_ASYNC_PF_INT I believe. -- Vitaly
