From af95cafb877f563ed39ce8a5b2abe227dd5312fa Mon Sep 17 00:00:00 2001 From: Eduardo Habkost Date: Thu, 22 Aug 2019 19:52:10 -0300 Subject: [PATCH] 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 Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1741508 Signed-off-by: Eduardo Habkost Message-Id: <20190822225210.32541-1-ehabkost@redhat.com> Acked-by: Paolo Bonzini Signed-off-by: Eduardo Habkost --- target/i386/kvm.c | 14 ++++++++++++++ 1 file changed, 14 insertions(+) diff --git a/target/i386/kvm.c b/target/i386/kvm.c index 11b9c854b5..8c73438c67 100644 --- a/target/i386/kvm.c +++ b/target/i386/kvm.c @@ -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; } }