From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Michael S. Tsirkin" Subject: Re: [PATCH] kvm: better MWAIT emulation for guests Date: Wed, 15 Mar 2017 20:29:23 +0200 Message-ID: <20170315202049-mutt-send-email-mst@kernel.org> References: <1489098555-23856-1-git-send-email-mst@redhat.com> <20170313154618.GA4547@potion> <20170313180046-mutt-send-email-mst@kernel.org> <20170313193910.GB4547@potion> <20170313215016-mutt-send-email-mst@kernel.org> <20170313214355.GC4547@potion> <20170315181425.GA2239@HEDWIG.INI.CMU.EDU> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Radim =?utf-8?B?S3LEjW3DocWZ?= , linux-kernel@vger.kernel.org, Paolo Bonzini , Jonathan Corbet , Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , x86@kernel.org, kvm@vger.kernel.org, linux-doc@vger.kernel.org To: "Gabriel L. Somlo" Return-path: Content-Disposition: inline In-Reply-To: <20170315181425.GA2239@HEDWIG.INI.CMU.EDU> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org On Wed, Mar 15, 2017 at 02:14:26PM -0400, Gabriel L. Somlo wrote: > Michael, > > I tested this on OS X 10.7 (Lion), the last version that doesn't check > CPUID for MWAIT support. > > I used the latest kvm from git://git.kernel.org/pub/scm/virt/kvm/kvm.git > first as-is, then with your v2 MWAIT patch applied. > > Single-(V)CPU guest works as expected (but then again, single-vcpu > guests worked even back when I tried emulating MWAIT the same as HLT). > > When I try starting a SMP guest (with "-smp 4,cores=2"), the guest OS > hangs after generating some output in text/verbose boot mode -- I gave > up waiting for it after about 5 minutes. Works fine before your patch, > which leads me to suspect that, as I feared, MWAIT doesn't wake > immediately upon another VCPU writing to the MONITOR-ed memory location. > > Tangentially, I remember back in the days of OS X 10.7, the > alternative to exiting guest mode and emulating MWAIT and MONITOR as > NOPs was to allow them both to run in guest mode. > > While poorly documented by Intel at the time, MWAIT at L>0 effectively > behaves as a NOP (i.e., doesn't actually put the physical core into > low-power mode, because doing that would allow a guest to effectively > DOS the host hardware). Thanks for the testing, interesting. Testing with Linux guest seems to show it works. This could be an interrupt thing not a monitor thing. Question: does your host CPU have this in its MWAIT leaf? Bit 01: Supports treating interrupts as break-event for MWAIT, even when interrupts disabled We really should check that before enabling, I'll add that. > > Given how unusual it is for a guest to use MONITOR/MWAIT in the first > place, what's wrong with leaving it all as is (i.e., emulated as NOP)? > > Thanks, > --Gabriel I'm really looking into ways to use mwait within Linux guests, this is just a building block that should help Mac OSX as a side effect (and we do not want it broken if at all possible). -- MST