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i386: Omit all-zeroes entries from KVM CPUID table

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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>
Message-Id: <20190822225210.32541-1-ehabkost@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
This commit is contained in:
Eduardo Habkost 2019-08-22 19:52:10 -03:00
parent 76ecd7a514
commit af95cafb87
1 changed files with 14 additions and 0 deletions
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14
target/i386/kvm.c
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@ -1567,6 +1567,13 @@ int kvm_arch_init_vcpu(CPUState *cs)
c->function = i; c->function = i;
c->flags = 0; c->flags = 0;
cpu_x86_cpuid(env, i, 0, &c->eax, &c->ebx, &c->ecx, &c->edx); 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; break;
} }
} }
@ -1631,6 +1638,13 @@ int kvm_arch_init_vcpu(CPUState *cs)
c->function = i; c->function = i;
c->flags = 0; c->flags = 0;
cpu_x86_cpuid(env, i, 0, &c->eax, &c->ebx, &c->ecx, &c->edx); 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; break;
} }
} }