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[91.219.240.2]) by smtp.gmail.com with ESMTPSA id z15sm27228395wrv.94.2020.09.22.08.38.12 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 22 Sep 2020 08:38:13 -0700 (PDT) From: Vitaly Kuznetsov To: Eduardo Habkost , qemu-devel@nongnu.org Cc: "Dr. David Alan Gilbert" , Eduardo Habkost , 1896263@bugs.launchpad.net, kvm@vger.kernel.org, Paolo Bonzini , Richard Henderson , Marcelo Tosatti , Laurent Vivier , Stefan Hajnoczi Subject: Re: [PATCH] i386: Don't try to set MSR_KVM_ASYNC_PF_EN if kernel-irqchip=off In-Reply-To: <20200922151455.1763896-1-ehabkost@redhat.com> References: <20200922151455.1763896-1-ehabkost@redhat.com> Date: Tue, 22 Sep 2020 17:38:12 +0200 Message-ID: <87v9g5es9n.fsf@vitty.brq.redhat.com> MIME-Version: 1.0 Content-Type: text/plain Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org Eduardo Habkost 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 > --- > 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