As a preparation to making Hyper-V TSC page suitable for vDSO move
the TSC page reading logic to asm/mshyperv.h. While on it, do the
following
- Document the reading algorithm.
- Simplify the code a bit.
- Add explicit barriers to prevent re-ordering (we need to read sequence
stricktly before and after)
- Use mul_u64_u64_shr() instead of assembly. I checked and on x86_64
gcc generates a single 'mul' instruction.
Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
---
arch/x86/hyperv/hv_init.c | 36 ++++-------------------------
arch/x86/include/asm/mshyperv.h | 50 +++++++++++++++++++++++++++++++++++++++++
2 files changed, 54 insertions(+), 32 deletions(-)
diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
index 0ce8485..77fec0c 100644
--- a/arch/x86/hyperv/hv_init.c
+++ b/arch/x86/hyperv/hv_init.c
@@ -38,39 +38,11 @@ struct ms_hyperv_tsc_page *hv_get_tsc_page(void)
static u64 read_hv_clock_tsc(struct clocksource *arg)
{
- u64 current_tick;
+ u64 current_tick = hv_read_tsc_page(tsc_pg);
+
+ if (current_tick == U64_MAX)
+ rdmsrl(HV_X64_MSR_TIME_REF_COUNT, current_tick);
- if (tsc_pg->tsc_sequence != 0) {
- /*
- * Use the tsc page to compute the value.
- */
-
- while (1) {
- u64 tmp;
- u32 sequence = tsc_pg->tsc_sequence;
- u64 cur_tsc;
- u64 scale = tsc_pg->tsc_scale;
- s64 offset = tsc_pg->tsc_offset;
-
- rdtscll(cur_tsc);
- /* current_tick = ((cur_tsc *scale) >> 64) + offset */
- asm("mulq %3"
- : "=d" (current_tick), "=a" (tmp)
- : "a" (cur_tsc), "r" (scale));
-
- current_tick += offset;
- if (tsc_pg->tsc_sequence == sequence)
- return current_tick;
-
- if (tsc_pg->tsc_sequence != 0)
- continue;
- /*
- * Fallback using MSR method.
- */
- break;
- }
- }
- rdmsrl(HV_X64_MSR_TIME_REF_COUNT, current_tick);
return current_tick;
}
diff --git a/arch/x86/include/asm/mshyperv.h b/arch/x86/include/asm/mshyperv.h
index 14dd92c..ddd071c 100644
--- a/arch/x86/include/asm/mshyperv.h
+++ b/arch/x86/include/asm/mshyperv.h
@@ -175,6 +175,56 @@ void hyperv_cleanup(void);
#endif
#ifdef CONFIG_HYPERV_TSCPAGE
struct ms_hyperv_tsc_page *hv_get_tsc_page(void);
+static inline u64 hv_read_tsc_page(const struct ms_hyperv_tsc_page *tsc_pg)
+{
+ u64 scale, offset, current_tick, cur_tsc;
+ u32 sequence;
+
+ /*
+ * The protocol for reading Hyper-V TSC page is specified in Hypervisor
+ * Top-Level Functional Specification ver. 3.0 and above. To get the
+ * reference time we must do the following:
+ * - READ ReferenceTscSequence
+ * A special '0' value indicates the time source is unreliable and we
+ * need to use something else. The currently published specification
+ * versions (up to 4.0b) contain a mistake and wrongly claim '-1'
+ * instead of '0' as the special value, see commit c35b82ef0294.
+ * - ReferenceTime =
+ * ((RDTSC() * ReferenceTscScale) >> 64) + ReferenceTscOffset
+ * - READ ReferenceTscSequence again. In case its value has changed
+ * since our first reading we need to discard ReferenceTime and repeat
+ * the whole sequence as the hypervisor was updating the page in
+ * between.
+ */
+ while (1) {
+ sequence = tsc_pg->tsc_sequence;
+ if (!sequence)
+ break;
+ /*
+ * Make sure we read sequence before we read other values from
+ * TSC page.
+ */
+ virt_rmb();
+
+ scale = tsc_pg->tsc_scale;
+ offset = tsc_pg->tsc_offset;
+ rdtscll(cur_tsc);
+
+ current_tick = mul_u64_u64_shr(cur_tsc, scale, 64) + offset;
+
+ /*
+ * Make sure we read sequence after we read all other values
+ * from TSC page.
+ */
+ virt_rmb();
+
+ if (tsc_pg->tsc_sequence == sequence)
+ return current_tick;
+ }
+
+ return U64_MAX;
+}
+
#else
static inline struct ms_hyperv_tsc_page *hv_get_tsc_page(void)
{
--
2.9.3
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