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

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Date:   Wed, 27 Sep 2017 16:26:10 +0100
From:   Joao Martins <joao.m.martins@...cle.com>
To:     Boris Ostrovsky <boris.ostrovsky@...cle.com>
Cc:     linux-kernel@...r.kernel.org, xen-devel@...ts.xenproject.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Juergen Gross <jgross@...e.com>,
        Andy Lutomirski <luto@...capital.net>
Subject: Re: [PATCH v4 2/3] x86/xen/time: setup vcpu 0 time info page

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. 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;
>> +}
>> +
>