From 668fffa3f838edfcb1679f842f7ef1afa61c3e9a Mon Sep 17 00:00:00 2001 From: Michael S. Tsirkin Date: Fri, 21 Apr 2017 12:27:17 +0200 Subject: 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" Signed-off-by: Michael S. Tsirkin [agraf: fix amd, change commit message] Signed-off-by: Alexander Graf Signed-off-by: Paolo Bonzini --- Documentation/virtual/kvm/api.txt | 9 +++++++++ 1 file changed, 9 insertions(+) (limited to 'Documentation/virtual/kvm') 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). -- cgit v1.2.3