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

Radim Krčmář
2017-03-16T16:01:58Z

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.

---
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