[Minios-devel] [UNIKRAFT PATCH] plat/kvm/x86: retrieve TSC frequency from hypervisor if available

[Hugo Lefeuvre <hugo.lefeuvre@xxxxxxxxx>]

The TSC clock frequency is currently estimated using the i8254 timer over a
period of 0.1s. This solution is undesirable because it delays the boot.

Hypervisors advertise the TSC clock frequency via the hypervisor generic
cpuid timing information leaf 0x40000010 [0]. This feature is available in
QEMU/KVM since 2.9 with -cpu options vmware-cpuid-freq=on and +invtsc.

Retrieve TSC clock frequency via cpuid. If unavailable, fall back to manual
calibration.

[0] https://lwn.net/Articles/301888/

Signed-off-by: Hugo Lefeuvre <hugo.lefeuvre@xxxxxxxxx>
---
 plat/kvm/x86/tscclock.c | 34 +++++++++++++++++++++++++---------
 1 file changed, 25 insertions(+), 9 deletions(-)

diff --git a/plat/kvm/x86/tscclock.c b/plat/kvm/x86/tscclock.c
index 86db814..727164a 100644
--- a/plat/kvm/x86/tscclock.c
+++ b/plat/kvm/x86/tscclock.c
@@ -216,7 +216,8 @@ __u64 tscclock_monotonic(void)
  */
 int tscclock_init(void)
 {
-       __u64 tsc_freq, rtc_boot;
+       __u64 tsc_freq = 0, rtc_boot;
+       __u32 eax, ebx, ecx, edx;
 
        /* Initialise i8254 timer channel 0 to mode 2 at CONFIG_HZ frequency */
        outb(TIMER_MODE, TIMER_SEL0 | TIMER_RATEGEN | TIMER_16BIT);
@@ -230,15 +231,30 @@ int tscclock_init(void)
        rtc_boot = rtc_gettimeofday();
 
        /*
-        * Calculate TSC frequency by calibrating against an 0.1s delay
-        * using the i8254 timer.
-        * TODO: Find a more elegant solution that does not require us to
-        * to delay the boot for 100ms. Does KVM provides us a pre-calculated
-        * TSC value?
+        * Attempt to retrieve TSC frequency via the hypervisor generic cpuid
+        * timing information leaf. 0x40000010 returns the (virtual) TSC
+        * frequency in kHz, or 0 if the feature is not supported by the
+        * hypervisor.
         */
-       tsc_base = rdtsc();
-       i8254_delay(100000);
-       tsc_freq = (rdtsc() - tsc_base) * 10;
+       cpuid(0x40000000, 0, &eax, &ebx, &ecx, &edx);
+       if (eax >= 0x40000010) {
+               uk_pr_info("Retrieving TSC clock frequency from hypervisor\n");
+               cpuid(0x40000010, 0, &eax, &ebx, &ecx, &edx);
+               tsc_freq = eax * 1000;
+       }
+
+       /*
+        * If we could not retrieve the TSC frequency from the hypervisor,
+        * calibrate against an 0.1s delay using the i8254 timer. This is
+        * undesirable as it delays the boot sequence.
+        */
+       if (!tsc_freq) {
+               uk_pr_info("Calibrating TSC clock against i8254 timer\n");
+               tsc_base = rdtsc();
+               i8254_delay(100000);
+               tsc_freq = (rdtsc() - tsc_base) * 10;
+       }
+
        uk_pr_info("Clock source: TSC, frequency estimate is %llu Hz\n",
                   (unsigned long long) tsc_freq);
 
-- 
2.7.4


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