* [PATCH] i386: Omit all-zeroes entries from KVM CPUID table
@ 2019-08-22 22:52 Eduardo Habkost
2019-08-22 23:02 ` Paolo Bonzini
0 siblings, 1 reply; 2+ messages in thread
From: Eduardo Habkost @ 2019-08-22 22:52 UTC (permalink / raw)
To: qemu-devel
Cc: Paolo Bonzini, kvm, Eduardo Habkost, Marcelo Tosatti,
Igor Mammedov, Richard Henderson, Yumei Huang
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
^ permalink raw reply related [flat|nested] 2+ messages in thread
* Re: [PATCH] i386: Omit all-zeroes entries from KVM CPUID table
2019-08-22 22:52 [PATCH] i386: Omit all-zeroes entries from KVM CPUID table Eduardo Habkost
@ 2019-08-22 23:02 ` Paolo Bonzini
0 siblings, 0 replies; 2+ messages in thread
From: Paolo Bonzini @ 2019-08-22 23:02 UTC (permalink / raw)
To: Eduardo Habkost, qemu-devel
Cc: kvm, Marcelo Tosatti, Igor Mammedov, Richard Henderson, Yumei Huang
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;
> }
> }
>
^ permalink raw reply [flat|nested] 2+ messages in thread
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2019-08-22 23:02 ` Paolo Bonzini
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