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

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Message-ID: <20170316160157.GN14081@potion>
Date:   Thu, 16 Mar 2017 17:01:58 +0100
From:   Radim Krčmář <rkrcmar@...hat.com>
To:     "Gabriel L. Somlo" <gsomlo@...il.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        linux-kernel@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-doc@...r.kernel.org
Subject: Re: [PATCH v5 untested] kvm: better MWAIT emulation for guests

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