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

Radim Krčmář
2017-03-16T17:22:44Z

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
time TIMEOUT=20 ./x86-run x86/mwait.flat -append '0 0 1' -smp 2
time TIMEOUT=20 ./x86-run x86/mwait.flat -append '0 0 0' -smp 2

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

Thanks.

---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++;