kvm: better MWAIT emulation for guests

Guests that are heavy on futexes end up IPI'ing each other a lot. That
can lead to significant slowdowns and latency increase for those guests
when running within KVM.

If only a single guest is needed on a host, we have a lot of spare host
CPU time we can throw at the problem. Modern CPUs implement a feature
called "MWAIT" which allows guests to wake up sleeping remote CPUs without
an IPI - thus without an exit - at the expense of never going out of guest
context.

The decision whether this is something sensible to use should be up to the
VM admin, so to user space. We can however allow MWAIT execution on systems
that support it properly hardware wise.

This patch adds a CAP to user space and a KVM cpuid leaf to indicate
availability of native MWAIT execution. With that enabled, the worst a
guest can do is waste as many cycles as a "jmp ." would do, so it's not
a privilege problem.

We consciously do *not* expose the feature in our CPUID bitmap, as most
people will want to benefit from sleeping vCPUs to allow for over commit.

Reported-by: "Gabriel L. Somlo" <gsomlo@gmail.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
[agraf: fix amd, change commit message]
Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 36 insertions, 0 deletions

diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
index e8ff3e4ce38a..612067074905 100644
--- a/arch/x86/kvm/x86.h
+++ b/arch/x86/kvm/x86.h
@@ -1,6 +1,8 @@
 #ifndef ARCH_X86_KVM_X86_H
 #define ARCH_X86_KVM_X86_H
 
+#include <asm/processor.h>
+#include <asm/mwait.h>
 #include <linux/kvm_host.h>
 #include <asm/pvclock.h>
 #include "kvm_cache_regs.h"
@@ -212,4 +214,38 @@ static inline u64 nsec_to_cycles(struct kvm_vcpu *vcpu, u64 nsec)
 	    __rem;						\
 	 })
 
+static inline bool kvm_mwait_in_guest(void)
+{
+	unsigned int eax, ebx, ecx, edx;
+
+	if (!cpu_has(&boot_cpu_data, X86_FEATURE_MWAIT))
+		return false;
+
+	switch (boot_cpu_data.x86_vendor) {
+	case X86_VENDOR_AMD:
+		/* All AMD CPUs have a working MWAIT implementation */
+		return true;
+	case X86_VENDOR_INTEL:
+		/* Handle Intel below */
+		break;
+	default:
+		return false;
+	}
+
+	/*
+	 * Intel CPUs without CPUID5_ECX_INTERRUPT_BREAK are problematic as
+	 * they would allow guest to stop the CPU completely by disabling
+	 * interrupts then invoking MWAIT.
+	 */
+	if (boot_cpu_data.cpuid_level < CPUID_MWAIT_LEAF)
+		return false;
+
+	cpuid(CPUID_MWAIT_LEAF, &eax, &ebx, &ecx, &edx);
+
+	if (!(ecx & CPUID5_ECX_INTERRUPT_BREAK))
+		return false;
+
+	return true;
+}
+
 #endif