On Tue, Sep 22, 2020 at 06:42:17PM +0200, Vitaly Kuznetsov wrote: > Eduardo Habkost <ehabkost@xxxxxxxxxx> writes: > > > On Tue, Sep 22, 2020 at 05:38:12PM +0200, Vitaly Kuznetsov wrote: > >> 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. [...] > > > > Were guests able to set MSR_KVM_ASYNC_PF_EN to non-zero with > > kernel-irqchip=off on hosts running Linux <= 5.7? > > lapic_in_kernel() check appeared in kernel with the following commit: > > commit 9d3c447c72fb2337ca39f245c6ae89f2369de216 > Author: Wanpeng Li <wanpengli@xxxxxxxxxxx> > Date: Mon Jun 29 18:26:31 2020 +0800 > > KVM: X86: Fix async pf caused null-ptr-deref > > which was post-interrupt-based-APF. I *think* it was OK to enable APF > with !lapic_in_kernel() before (at least I don't see what would not > allow that). If it was possible, did KVM break live migration of kernel-irqchip=off guests that enabled APF? This would mean my patch is replacing a crash with a silent migration bug. Not nice either way. > > > 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? > > (Sarcasm: if disallowing 'kernel-irqchip=off' is not an option, then) I'm working on it. :-) > yes, we probably can, but kvm-asyncpf-int=on is the default we have so > we can't just implicitly change it underneath or migration will break... kvm-asyncpf-int wasn't merged yet, was it? This means we don't have compatibility issues to care about. -- Eduardo
