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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 /Documentation/virtual
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 'Documentation/virtual')
-rw-r--r--Documentation/virtual/kvm/api.txt9
1 files changed, 9 insertions, 0 deletions
diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
index e60be91d8036..dc674c2b8b31 100644
--- a/Documentation/virtual/kvm/api.txt
+++ b/Documentation/virtual/kvm/api.txt
@@ -4111,3 +4111,12 @@ reserved.
2: MIPS64 or microMIPS64 with access to all address segments.
Both registers and addresses are 64-bits wide.
It will be possible to run 64-bit or 32-bit guest code.
+
+8.8 KVM_CAP_X86_GUEST_MWAIT
+
+Architectures: x86
+
+This capability indicates that guest using memory monotoring instructions
+(MWAIT/MWAITX) to stop the virtual CPU will not cause a VM exit. As such time
+spent while virtual CPU is halted in this way will then be accounted for as
+guest running time on the host (as opposed to e.g. HLT).