Re: [PATCH v5 untested] kvm: better MWAIT emulation for guests

From: "Radim Krčmář" <rkrcmar@redhat.com>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: "Gabriel L. Somlo" <gsomlo@gmail.com>,
	linux-kernel@vger.kernel.org, Paolo Bonzini <pbonzini@redhat.com>,
	Jonathan Corbet <corbet@lwn.net>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
	x86@kernel.org, Joerg Roedel <joro@8bytes.org>,
	kvm@vger.kernel.org, linux-doc@vger.kernel.org
Subject: Re: [PATCH v5 untested] kvm: better MWAIT emulation for guests
Date: Tue, 21 Mar 2017 17:58:32 +0100	[thread overview]
Message-ID: <20170321165831.GC25540@potion> (raw)
In-Reply-To: <20170321052102-mutt-send-email-mst@kernel.org>

2017-03-21 05:22+0200, Michael S. Tsirkin:
> On Fri, Mar 17, 2017 at 09:23:56AM -0400, Gabriel L. Somlo wrote:
>> OK, now on to Radim's test, on the MacPro1,1:
>> 
>> [kvm-unit-tests]$ time TIMEOUT=20 ./x86-run x86/mwait.flat -append '240 1 1'
>> timeout -k 1s --foreground 20 qemu-kvm -nodefaults -enable-kvm -device pc-testdev -device isa-debug-exit,iobase=0xf4,iosize=0x4 -vnc none -serial stdio -device pci-testdev -kernel x86/mwait.flat -append 240 1 1
>> enabling apic
>> PASS: resumed from mwait 10000 times
>> SUMMARY: 1 tests
>> 
>> real    0m0.746s
>> user    0m0.542s
>> sys     0m0.215s
>> [kvm-unit-tests]$ time TIMEOUT=20 ./x86-run x86/mwait.flat -append '240 1 0'
>> timeout -k 1s --foreground 20 qemu-kvm -nodefaults -enable-kvm -device pc-testdev -device isa-debug-exit,iobase=0xf4,iosize=0x4 -vnc none -serial stdio -device pci-testdev -kernel x86/mwait.flat -append 240 1 0
>> enabling apic
>> PASS: resumed from mwait 10000 times
>> SUMMARY: 1 tests
>> 
>> real    0m0.743s
>> user    0m0.528s
>> sys     0m0.226s
>> [kvm-unit-tests]$ time TIMEOUT=20 ./x86-run x86/mwait.flat -append '240 1 1' -smp 2
>> timeout -k 1s --foreground 20 qemu-kvm -nodefaults -enable-kvm -device pc-testdev -device isa-debug-exit,iobase=0xf4,iosize=0x4 -vnc none -serial stdio -device pci-testdev -kernel x86/mwait.flat -append 240 1 1 -smp 2
>> enabling apic
>> enabling apic
>> FAIL: resumed from mwait 10150 times
>> SUMMARY: 1 tests, 1 unexpected failures
>> 
>> real    0m0.745s
>> user    0m0.545s
>> sys     0m0.214s
>> [kvm-unit-tests]$ time TIMEOUT=20 ./x86-run x86/mwait.flat -append '240 1 0' -smp 2
>> timeout -k 1s --foreground 20 qemu-kvm -nodefaults -enable-kvm -device pc-testdev -device isa-debug-exit,iobase=0xf4,iosize=0x4 -vnc none -serial stdio -device pci-testdev -kernel x86/mwait.flat -append 240 1 0 -smp 2
>> enabling apic
>> enabling apic
>> FAIL: resumed from mwait 10116 times
>> SUMMARY: 1 tests, 1 unexpected failures
>> 
>> real    0m0.744s
>> user    0m0.541s
>> sys     0m0.217s
>> 
>> HTH,
>> --Gabriel
> 
> Weird. How can it go above 10000? Radim - any idea?

In '-smp 2', the writing VCPU always does 10000 wakeups by writing into
monitored memory, but the mwaiting VCPU can be also woken up by host
interrupts, which might add a few exits depending on timing.

I didn't spend much time in making the PASS/FAIL mean much, or ensuring
that we only get 10000 wakeups ... it is nothing to be worried about.

Hint 240 behaves as nop even on my system, so I still don't find
anything insane on that machine (if OS X is exluded) ...