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

Gabriel L. Somlo
2017-03-16T17:41:50Z

On Thu, Mar 16, 2017 at 07:27:34PM +0200, Michael S. Tsirkin wrote:
> On Thu, Mar 16, 2017 at 12:47:50PM -0400, Gabriel L. Somlo wrote:
> > On Thu, Mar 16, 2017 at 05:01:58PM +0100, Radim Krčmář wrote:
> > > 2017-03-16 16:35+0100, Radim Krčmář:
> > > > 2017-03-16 10:58-0400, Gabriel L. Somlo:
> > > >> The intel manual said the same thing back in 2010 as well. However,
> > > >> regardless of how any flags were set, interrupt-window exiting or not,
> > > >> "normal" L1 MWAIT behavior was that it woke up immediately regardless.
> > > >> Remember, never going to sleep is still correct ("normal" ?) behavior
> > > >> per the ISA definition of MWAIT :)
> > > >
> > > > I'll write a simple kvm-unit-test to better understand why it is broken
> > > > for you ...
> > >
> > > Please get git://git.kernel.org/pub/scm/virt/kvm/kvm-unit-tests.git
> > >
> > > and try this, thanks!
> > >
> > > ---8<---
> > > x86/mwait: crappy test
> > >
> > > `./configure && make` to build it, then follow the comment in code to
> > > try few cases.
> >
> > kvm-unit-tests]$ time TIMEOUT=20 ./x86-run x86/mwait.flat -append '0 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 0 1 1
> > enabling apic
> > PASS: resumed from mwait 10000 times
> > SUMMARY: 1 tests
> >
> > real 0m10.564s
> > user 0m10.339s
> > sys 0m0.225s
> >
> >
> > and
> >
> > kvm-unit-tests]$ time TIMEOUT=20 ./x86-run x86/mwait.flat -append '0 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 0 1 0
> > enabling apic
> > PASS: resumed from mwait 10000 times
> > SUMMARY: 1 tests
> >
> > real 0m0.746s
> > user 0m0.555s
> > sys 0m0.200s
> >
> > Both of these with Michael's v5 patch applied, on the MacPro1,1.
>
> Would it make sense to try to set ECX to 0? 0 0 1 and 0 0 0.

$ time TIMEOUT=20 ./x86-run x86/mwait.flat -append '0 0 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 0 0 1
enabling apic
PASS: resumed from mwait 10000 times
SUMMARY: 1 tests

real 0m10.567s
user 0m10.367s
sys 0m0.210s

$ time TIMEOUT=20 ./x86-run x86/mwait.flat -append '0 0 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 0 0 0
enabling apic
PASS: resumed from mwait 10000 times
SUMMARY: 1 tests

real 0m10.549s
user 0m10.352s
sys 0m0.206s

Both took 10 seconds.

>
> > Similar behavior (0 1 1 takes 10 seconds, 0 1 0 returns immediately)
> > on the macbook air.
> >
> > If I revert to the original (nop-emulated MWAIT) kvm source, I get
> > both versions to return immediately.
> >
> > HTH,
> > --Gabriel
> >
> >
> >
> > >
> > > ---
> > > x86/Makefile.common | 1 +
> > > x86/mwait.c | 41 +++++++++++++++++++++++++++++++++++++++++
> > > 2 files changed, 42 insertions(+)
> > > create mode 100644 x86/mwait.c
> > >
> > > diff --git a/x86/Makefile.common b/x86/Makefile.common
> > > index 1dad18ba26e1..1e708a6acd39 100644
> > > --- a/x86/Makefile.common
> > > +++ b/x86/Makefile.common
> > > @@ -46,6 +46,7 @@ tests-common = $(TEST_DIR)/vmexit.flat $(TEST_DIR)/tsc.flat \
> > > $(TEST_DIR)/tsc_adjust.flat $(TEST_DIR)/asyncpf.flat \
> > > $(TEST_DIR)/init.flat $(TEST_DIR)/smap.flat \
> > > $(TEST_DIR)/hyperv_synic.flat $(TEST_DIR)/hyperv_stimer.flat \
> > > + $(TEST_DIR)/mwait.flat \
> > >
> > > ifdef API
> > > tests-common += api/api-sample
> > > diff --git a/x86/mwait.c b/x86/mwait.c
> > > new file mode 100644
> > > index 000000000000..c21dab5cc97d
> > > --- /dev/null
> > > +++ b/x86/mwait.c
> > > @@ -0,0 +1,41 @@
> > > +#include "vm.h"
> > > +
> > > +#define TARGET_RESUMES 10000
> > > +volatile unsigned page[4096 / 4];
> > > +
> > > +/*
> > > + * Execute
> > > + * time TIMEOUT=20 ./x86-run x86/mwait.flat -append '0 1 1'
> > > + * (first two arguments are eax and ecx for MWAIT, the third is FLAGS.IF bit)
> > > + * I assume you have 1000 Hz scheduler, so the test should take about 10
> > > + * seconds to run if mwait works (host timer interrupts will kick mwait).
> > > + *
> > > + * If you get far less, then mwait is just nop, as in the case of
> > > + *
> > > + * time TIMEOUT=20 ./x86-run x86/mwait.flat -append '0 1 0'
> > > + *
> > > + * All other combinations of arguments should take 10 seconds.
> > > + * Getting killed by the TIMEOUT most likely means that you have different HZ,
> > > + * but could also be a bug ...
> > > + */
> > > +int main(int argc, char **argv)
> > > +{
> > > + uint32_t eax = atol(argv[1]);
> > > + uint32_t ecx = atol(argv[2]);
> > > + bool sti = atol(argv[3]);
> > > + unsigned resumes = 0;
> > > +
> > > + if (sti)
> > > + asm volatile ("sti");
> > > + else
> > > + asm volatile ("cli");
> > > +
> > > + while (resumes < TARGET_RESUMES) {
> > > + asm volatile("monitor" :: "a" (page), "c" (0), "d" (0));
> > > + asm volatile("mwait" :: "a" (eax), "c" (ecx));
> > > + resumes++;
> > > + }
> > > +
> > > + report("resumed from mwait %u times", resumes == TARGET_RESUMES, resumes);
> > > + return report_summary();
> > > +}
> > > --
> > > 2.11.0
> > >