x86/hyperv: Move TSC reading method to asm/mshyperv.h

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 READ_ONCE() to not rely on 'volatile'.
- Add explicit barriers to prevent re-ordering (we need to read sequence
  strictly before and after)
- Use mul_u64_u64_shr() instead of assembly, gcc generates a single 'mul'
  instruction on x86_64 anyway.

[ tglx: Simplified the loop ]

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: Stephen Hemminger <sthemmin@microsoft.com>
Cc: Haiyang Zhang <haiyangz@microsoft.com>
Cc: Dexuan Cui <decui@microsoft.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: devel@linuxdriverproject.org
Cc: "K. Y. Srinivasan" <kys@microsoft.com>
Cc: virtualization@lists.linux-foundation.org
Link: http://lkml.kernel.org/r/20170303132142.25595-3-vkuznets@redhat.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
index bb1ea58ecd7a4e55a2064c277cafd56867f408ad..7f5152356a59eadd536d0811e96a97500875b83a 100644 (file)
--- 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 d324dce133a1f6fd4d34548a786938cf29ea1cea..fba1007139243b21081b6bfa114bed588e6f4e8e 100644 (file)
--- a/arch/x86/include/asm/mshyperv.h
+++ b/arch/x86/include/asm/mshyperv.h
@@ -178,6 +178,52 @@ 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)
 {