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

[Eduardo Habkost]

On Tue, Sep 22, 2020 at 05:38:12PM +0200, Vitaly Kuznetsov wrote:
> 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. [...]

Were guests able to set MSR_KVM_ASYNC_PF_EN to non-zero with
kernel-irqchip=off on hosts running Linux <= 5.7?  I am assuming
kvm-asyncpf never worked with kernel-irqchip=off (and enabling it
by default with kernel-irqchip=off was a mistake).


>                         [...] 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.

Shouldn't we just disallow kvm-asyncpf-int=on if kernel-irqchip=off?

-- 
Eduardo