Re: [Xen-devel] [PATCH v4 2/3] x86/xen/time: setup vcpu 0 time info page

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 09/27/2017 11:26 AM, Joao Martins wrote:
> On 09/27/2017 03:40 PM, Boris Ostrovsky wrote:
>>> +static void xen_setup_vsyscall_time_info(void)
>>> +{
>>> +   struct vcpu_register_time_memory_area t;
>>> +   struct pvclock_vsyscall_time_info *ti;
>>> +   struct pvclock_vcpu_time_info *pvti;
>>> +   int ret;
>>> +
>>> +   pvti = &__this_cpu_read(xen_vcpu)->time;
>>> +
>>> +   /*
>>> +    * We check ahead on the primary time info if this
>>> +    * bit is supported hence speeding up Xen clocksource.
>>> +    */
>>> +   if (!(pvti->flags & PVCLOCK_TSC_STABLE_BIT))
>>> +           return;
>>> +
>>> +   pvclock_set_flags(PVCLOCK_TSC_STABLE_BIT);
>> Is it OK to have this flag set if anything below fails?
>>
> Yes - if anything below fails it will only affect userspace mapped page.

Then should it be set somewhere else, like in xen_time_init()?

-boris

>  What I
> do above is just allowing xen clocksource to use/check that bit (consequently
> speeding up sched_clock) given the necessary support is there in the master
> copy. The secondary copy (i.e. what's being set up below, mapped/used in vdso)
> has the same data from the master copy, just separate memory regions. The 
> checks
> below are just for the unlikely cases of failing to register the secondary 
> copy
> or if its content were to differ from master copy in future releases - and
> therefore we handle those more gracefully.
>
>> (I can see in the changelog that apparently at some point I've asked
>> about this at v1 but I can't remember/find what exactly it was)
>>
>>> +
>>> +   ti = (struct pvclock_vsyscall_time_info *)get_zeroed_page(GFP_KERNEL);
>>> +   if (!ti)
>>> +           return;
>>> +
>>> +   t.addr.v = &ti->pvti;
>>> +
>>> +   ret = HYPERVISOR_vcpu_op(VCPUOP_register_vcpu_time_memory_area, 0, &t);
>>> +   if (ret) {
>>> +           pr_notice("xen: VCLOCK_PVCLOCK not supported (err %d)\n", ret);
>>> +           free_page((unsigned long)ti);
>>> +           return;
>>> +   }
>>> +
>>> +   /*
>>> +    * If the check above succedded this one should too since it's the
>>> +    * same data on both primary and secondary time infos just different
>>> +    * memory regions. But we still check it in case hypervisor is buggy.
>>> +    */
>>> +   pvti = &ti->pvti;
>>> +   if (!(pvti->flags & PVCLOCK_TSC_STABLE_BIT)) {
>>> +           t.addr.v = NULL;
>>> +           ret = HYPERVISOR_vcpu_op(VCPUOP_register_vcpu_time_memory_area,
>>> +                                    0, &t);
>>> +           if (!ret)
>>> +                   free_page((unsigned long)ti);
>>> +
>>> +           pr_notice("xen: VCLOCK_PVCLOCK not supported (tsc unstable)\n");
>>> +           return;
>>> +   }
>>> +
>>> +   xen_clock = ti;
>>> +   pvclock_set_pvti_cpu0_va(xen_clock);
>>> +
>>> +   xen_clocksource.archdata.vclock_mode = VCLOCK_PVCLOCK;
>>> +}
>>> +


_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
https://lists.xen.org/xen-devel