From 0733379b512ce36ba0b10942f9597b74f579f063 Mon Sep 17 00:00:00 2001 From: Vitaly Kuznetsov Date: Fri, 3 Mar 2017 14:21:41 +0100 Subject: 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 Cc: Stephen Hemminger Cc: Haiyang Zhang Cc: Dexuan Cui Cc: Andy Lutomirski Cc: devel@linuxdriverproject.org Cc: "K. Y. Srinivasan" Cc: virtualization@lists.linux-foundation.org Link: http://lkml.kernel.org/r/20170303132142.25595-3-vkuznets@redhat.com Signed-off-by: Thomas Gleixner --- arch/x86/include/asm/mshyperv.h | 46 +++++++++++++++++++++++++++++++++++++++++ 1 file changed, 46 insertions(+) (limited to 'arch/x86/include/asm/mshyperv.h') diff --git a/arch/x86/include/asm/mshyperv.h b/arch/x86/include/asm/mshyperv.h index d324dce133a1..fba100713924 100644 --- 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) { -- cgit v1.2.3-55-g7522