Commit af95cafb authored Aug 22, 2019 by Eduardo Habkost

i386: Omit all-zeroes entries from KVM CPUID table



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>

parent 76ecd7a5

target/i386/kvm.c

+14 −0

Original line number	Diff line number	Diff line
		@@ -1567,6 +1567,13 @@ 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;
		}
		}
		@@ -1631,6 +1638,13 @@ 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;
		}
		}