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

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Eduardo Habkost <ehabkost@redhat.com> 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@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)) {

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