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

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Message-ID: <b38ff455-d225-67d5-cf20-047ea54abce4@oracle.com>
Date:   Wed, 27 Sep 2017 10:40:21 -0400
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Joao Martins <joao.m.martins@...cle.com>,
        linux-kernel@...r.kernel.org, xen-devel@...ts.xenproject.org
Cc:     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


> +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?

(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)

-boris

> +
> +	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;
> +}
> +