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

Gabriel L. Somlo
2017-03-16T16:47:50Z

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.

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
>