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 bb1ea58..7f51523 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 (tsc_pg->tsc_sequence != 0) {
- /*
- * Use the tsc page to compute the value.
- */
+ if (current_tick == U64_MAX)
+ rdmsrl(HV_X64_MSR_TIME_REF_COUNT, current_tick);
- 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;
}