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

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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>
This commit is contained in:
Vitaly Kuznetsov 2017-03-03 14:21:41 +01:00 committed by Thomas Gleixner
parent bd2a9adaad
commit 0733379b51
2 changed files with 49 additions and 31 deletions
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34
arch/x86/hyperv/hv_init.c
View File

@ -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;
}

46
arch/x86/include/asm/mshyperv.h
View File

@ -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)
{