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

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;
         }
     }
-- 
2.21.0

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

Acked-by: Paolo Bonzini <pbonzini@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;
>          }
>      }
>