> -----Original Message----- > From: Thomas Gleixner [mailto:tglx@xxxxxxxxxxxxx] > Sent: Thursday, February 9, 2017 9:08 AM > To: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> > Cc: x86@xxxxxxxxxx; Andy Lutomirski <luto@xxxxxxxxxxxxxx>; Ingo Molnar > <mingo@xxxxxxxxxx>; H. Peter Anvin <hpa@xxxxxxxxx>; KY Srinivasan > <kys@xxxxxxxxxxxxx>; Haiyang Zhang <haiyangz@xxxxxxxxxxxxx>; Stephen > Hemminger <sthemmin@xxxxxxxxxxxxx>; Dexuan Cui > <decui@xxxxxxxxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx; > devel@xxxxxxxxxxxxxxxxxxxxxx; virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx > Subject: Re: [PATCH 2/2] x86/vdso: Add VCLOCK_HVCLOCK vDSO clock read > method > > On Thu, 9 Feb 2017, Vitaly Kuznetsov wrote: > > +#ifdef CONFIG_HYPERV_TSCPAGE > > +static notrace u64 vread_hvclock(int *mode) > > +{ > > + const struct ms_hyperv_tsc_page *tsc_pg = > > + (const struct ms_hyperv_tsc_page *)&hvclock_page; > > + u64 sequence, scale, offset, current_tick, cur_tsc; > > + > > + while (1) { > > + sequence = READ_ONCE(tsc_pg->tsc_sequence); > > + if (!sequence) > > + break; > > + > > + scale = READ_ONCE(tsc_pg->tsc_scale); > > + offset = READ_ONCE(tsc_pg->tsc_offset); > > + rdtscll(cur_tsc); > > + > > + current_tick = mul_u64_u64_shr(cur_tsc, scale, 64) + offset; > > + > > + if (READ_ONCE(tsc_pg->tsc_sequence) == sequence) > > + return current_tick; > > That sequence stuff lacks still a sensible explanation. It's fundamentally > different from the sequence counting we do in the kernel, so documentation > for it is really required. The host is updating multiple fields in this shared TSC page and the sequence number is used to ensure that the guest sees a consistent set values published. If I remember correctly, Xen has a similar mechanism. K. Y > > Thanks, > > tglx _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
