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authorMichael S. Tsirkin <mst@redhat.com>2017-04-21 13:27:17 +0300
committerPaolo Bonzini <pbonzini@redhat.com>2017-04-21 13:50:28 +0300
commit668fffa3f838edfcb1679f842f7ef1afa61c3e9a (patch)
treeedf2ba4b34dfc5f3757d20e94c598829bbb48e77 /arch/x86/kvm/svm.c
parentdb2336a80489e7c3c7728cefd9be58fac5ecfb39 (diff)
downloadlinux-668fffa3f838edfcb1679f842f7ef1afa61c3e9a.tar.xz
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>
Diffstat (limited to 'arch/x86/kvm/svm.c')
-rw-r--r--arch/x86/kvm/svm.c7
1 files changed, 5 insertions, 2 deletions
diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index 1b203abf76e1..c41f03e5090a 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -1198,10 +1198,13 @@ static void init_vmcb(struct vcpu_svm *svm)
set_intercept(svm, INTERCEPT_CLGI);
set_intercept(svm, INTERCEPT_SKINIT);
set_intercept(svm, INTERCEPT_WBINVD);
- set_intercept(svm, INTERCEPT_MONITOR);
- set_intercept(svm, INTERCEPT_MWAIT);
set_intercept(svm, INTERCEPT_XSETBV);
+ if (!kvm_mwait_in_guest()) {
+ set_intercept(svm, INTERCEPT_MONITOR);
+ set_intercept(svm, INTERCEPT_MWAIT);
+ }
+
control->iopm_base_pa = iopm_base;
control->msrpm_base_pa = __pa(svm->msrpm);
control->int_ctl = V_INTR_MASKING_MASK;