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-rw-r--r--arch/x86/include/asm/mshyperv.h54
1 files changed, 54 insertions, 0 deletions
diff --git a/arch/x86/include/asm/mshyperv.h b/arch/x86/include/asm/mshyperv.h
index 7c9c895432a9..fba100713924 100644
--- a/arch/x86/include/asm/mshyperv.h
+++ b/arch/x86/include/asm/mshyperv.h
@@ -176,4 +176,58 @@ void hyperv_report_panic(struct pt_regs *regs);
bool hv_is_hypercall_page_setup(void);
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, 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.
+ */
+ do {
+ sequence = READ_ONCE(tsc_pg->tsc_sequence);
+ if (!sequence)
+ return U64_MAX;
+ /*
+ * Make sure we read sequence before we read other values from
+ * TSC page.
+ */
+ smp_rmb();
+
+ scale = READ_ONCE(tsc_pg->tsc_scale);
+ offset = READ_ONCE(tsc_pg->tsc_offset);
+ cur_tsc = rdtsc_ordered();
+
+ /*
+ * Make sure we read sequence after we read all other values
+ * from TSC page.
+ */
+ smp_rmb();
+
+ } while (READ_ONCE(tsc_pg->tsc_sequence) != sequence);
+
+ return mul_u64_u64_shr(cur_tsc, scale, 64) + offset;
+}
+
+#else
+static inline struct ms_hyperv_tsc_page *hv_get_tsc_page(void)
+{
+ return NULL;
+}
+#endif
#endif