From: Eduardo Habkost <ehabkost@redhat.com>
To: qemu-devel@nongnu.org
Cc: Paolo Bonzini <pbonzini@redhat.com>,
kvm@vger.kernel.org, Eduardo Habkost <ehabkost@redhat.com>,
Marcelo Tosatti <mtosatti@redhat.com>,
Igor Mammedov <imammedo@redhat.com>,
Richard Henderson <rth@twiddle.net>,
Yumei Huang <yuhuang@redhat.com>
Subject: [PATCH] i386: Omit all-zeroes entries from KVM CPUID table
Date: Thu, 22 Aug 2019 19:52:10 -0300 [thread overview]
Message-ID: <20190822225210.32541-1-ehabkost@redhat.com> (raw)
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
next reply other threads:[~2019-08-22 22:52 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-08-22 22:52 Eduardo Habkost [this message]
2019-08-22 23:02 ` [PATCH] i386: Omit all-zeroes entries from KVM CPUID table Paolo Bonzini
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