On Thu, Feb 9, 2017 at 12:45 PM, KY Srinivasan <kys@xxxxxxxxxxxxx> wrote: > > >> -----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. So what's the actual protocol? When the hypervisor updates the page, does it freeze all guest cpus? If not, how does it maintain atomicity? --Andy _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
