On 23/08/19 00:52, Eduardo Habkost wrote: > KVM has a 80-entry limit at KVM_SET_CPUID2. With the > introduction of CPUID[0x1F], it is now possible to hit this limit > with unusual CPU configurations, e.g.: > > $ ./x86_64-softmmu/qemu-system-x86_64 \ > -smp 1,dies=2,maxcpus=2 \ > -cpu EPYC,check=off,enforce=off \ > -machine accel=kvm > qemu-system-x86_64: kvm_init_vcpu failed: Argument list too long > > This happens because QEMU adds a lot of all-zeroes CPUID entries > for unused CPUID leaves. In the example above, we end up > creating 48 all-zeroes CPUID entries. > > KVM already returns all-zeroes when emulating the CPUID > instruction if an entry is missing, so the all-zeroes entries are > redundant. Skip those entries. This reduces the CPUID table > size by half while keeping CPUID output unchanged. > > Reported-by: Yumei Huang <yuhuang@xxxxxxxxxx> > Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1741508 > Signed-off-by: Eduardo Habkost <ehabkost@xxxxxxxxxx> > --- > target/i386/kvm.c | 14 ++++++++++++++ > 1 file changed, 14 insertions(+) Acked-by: Paolo Bonzini <pbonzini@xxxxxxxxxx> > diff --git a/target/i386/kvm.c b/target/i386/kvm.c > index 8023c679ea..4e3df2867d 100644 > --- a/target/i386/kvm.c > +++ b/target/i386/kvm.c > @@ -1529,6 +1529,13 @@ int kvm_arch_init_vcpu(CPUState *cs) > c->function = i; > c->flags = 0; > cpu_x86_cpuid(env, i, 0, &c->eax, &c->ebx, &c->ecx, &c->edx); > + if (!c->eax && !c->ebx && !c->ecx && !c->edx) { > + /* > + * KVM already returns all zeroes if a CPUID entry is missing, > + * so we can omit it and avoid hitting KVM's 80-entry limit. > + */ > + cpuid_i--; > + } > break; > } > } > @@ -1593,6 +1600,13 @@ int kvm_arch_init_vcpu(CPUState *cs) > c->function = i; > c->flags = 0; > cpu_x86_cpuid(env, i, 0, &c->eax, &c->ebx, &c->ecx, &c->edx); > + if (!c->eax && !c->ebx && !c->ecx && !c->edx) { > + /* > + * KVM already returns all zeroes if a CPUID entry is missing, > + * so we can omit it and avoid hitting KVM's 80-entry limit. > + */ > + cpuid_i--; > + } > break; > } > } >
