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

Gabriel L. Somlo
2017-03-16T17:39:30Z

On Thu, Mar 16, 2017 at 06:22:44PM +0100, Radim Krčmář wrote:
> 2017-03-16 12:47-0400, Gabriel L. Somlo:
> > 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.
> >
> > 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.
>
> Those look normal ... maybe MWAIT just ignores writes to the monitored
> area?
>
> Please apply the patch below and following and try:
>
> time TIMEOUT=20 ./x86-run x86/mwait.flat -append '0 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 0 1 1 -smp 2
enabling apic
enabling apic
PASS: resumed from mwait 10000 times
SUMMARY: 1 tests

real 0m0.758s
user 0m0.557s
sys 0m0.220s

> time TIMEOUT=20 ./x86-run x86/mwait.flat -append '0 0 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 0 0 1 -smp 2
enabling apic
enabling apic
PASS: resumed from mwait 10000 times
SUMMARY: 1 tests

real 0m0.748s
user 0m0.550s
sys 0m0.210s

> time TIMEOUT=20 ./x86-run x86/mwait.flat -append '0 0 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 0 0 0 -smp 2
enabling apic
enabling apic
PASS: resumed from mwait 10000 times
SUMMARY: 1 tests

real 0m0.745s
user 0m0.558s
sys 0m0.203s

>
> All of them should take rougly the same time as the NOP one,
>
> time TIMEOUT=20 ./x86-run x86/mwait.flat -append '0 1 0' -smp 2

They all *did* return fast, as you expected.

> ---8<---
> diff --git a/x86/mwait.c b/x86/mwait.c
> index c21dab5cc97d..ca38e7223596 100644
> --- a/x86/mwait.c
> +++ b/x86/mwait.c
> @@ -1,7 +1,9 @@
> #include "vm.h"
> +#include "smp.h"
>
> #define TARGET_RESUMES 10000
> volatile unsigned page[4096 / 4];
> +volatile unsigned resumes;
>
> /*
> * Execute
> @@ -18,19 +20,39 @@ volatile unsigned page[4096 / 4];
> * Getting killed by the TIMEOUT most likely means that you have different HZ,
> * but could also be a bug ...
> */
> +void writer(void *null)
> +{
> + int i;
> + unsigned old_resumes = 0, new_resumes;
> +
> + for (i = 0; i < TARGET_RESUMES; i++) {
> + (*page)++;
> +
> + while (old_resumes == (new_resumes = resumes))
> + pause();
> + old_resumes = new_resumes;
> + }
> +}
> +
> 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;
> + bool smp;
> +
> + smp_init();
> + smp = cpu_count() > 1;
> +
> + if (smp)
> + on_cpu_async(1, writer, NULL);
>
> if (sti)
> asm volatile ("sti");
> else
> asm volatile ("cli");
>
> - while (resumes < TARGET_RESUMES) {
> + while ((smp ? *page : resumes) < TARGET_RESUMES) {
> asm volatile("monitor" :: "a" (page), "c" (0), "d" (0));
> asm volatile("mwait" :: "a" (eax), "c" (ecx));
> resumes++;