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Commit 0733379b authored by Vitaly Kuznetsov's avatar Vitaly Kuznetsov Committed by Thomas Gleixner
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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: default avatarVitaly 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: default avatarThomas Gleixner <tglx@linutronix.de>
parent bd2a9ada

arch/x86/hyperv/hv_init.c

+4 −32
Original line number Diff line number Diff line
@@ -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;



	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;

	u64 current_tick = hv_read_tsc_page(tsc_pg);



			if (tsc_pg->tsc_sequence != 0)

				continue;

			/*

			 * Fallback using MSR method.

			 */

			break;

		}

	}

	if (current_tick == U64_MAX)

		rdmsrl(HV_X64_MSR_TIME_REF_COUNT, current_tick);



	return current_tick;

}

arch/x86/include/asm/mshyperv.h

+46 −0
Original line number Diff line number Diff line
@@ -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)

{