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 <yuhu...@redhat.com>
> Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1741508
> Signed-off-by: Eduardo Habkost <ehabk...@redhat.com>
> ---
> target/i386/kvm.c | 14 ++++++++++++++
> 1 file changed, 14 insertions(+)
Acked-by: Paolo Bonzini <pbonz...@redhat.com>
> 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;
> }
> }
>
