Re: [PATCH] xen: preserve native TSC speed during migration between identical hosts

From: "Jan Beulich" <JBeulich@suse.com>
To: Olaf Hering <olaf@aepfle.de>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>,
	Wei Liu <wei.liu2@citrix.com>,
	Ian Jackson <ian.jackson@eu.citrix.com>,
	xen-devel@lists.xen.org
Subject: Re: [PATCH] xen: preserve native TSC speed during migration between identical hosts
Date: Mon, 29 May 2017 08:50:23 -0600	[thread overview]
Message-ID: <592C514F020000780015D542@prv-mh.provo.novell.com> (raw)
In-Reply-To: <20170524142505.11460-1-olaf@aepfle.de>

>>> On 24.05.17 at 16:25, <olaf@aepfle.de> wrote:
> @@ -2024,6 +2029,13 @@ void tsc_set_info(struct domain *d,
>          d->arch.vtsc_offset = get_s_time() - elapsed_nsec;
>          d->arch.tsc_khz = gtsc_khz ?: cpu_khz;
>          set_time_scale(&d->arch.vtsc_to_ns, d->arch.tsc_khz * 1000);
> +        if (!opt_vtsc_tolerance) {
> +            tolerated = d->arch.tsc_khz == cpu_khz;
> +        } else {

Leaving aside the question of whether we want anything like this,
there are multiple coding style issues here (braces on their own
lines, blanks inside the parentheses of control statements). 

> +            khz_diff = cpu_khz > d->arch.tsc_khz ?
> +                       cpu_khz - d->arch.tsc_khz : d->arch.tsc_khz - cpu_khz;
> +            tolerated = khz_diff <= opt_vtsc_tolerance;
> +        }

These assignments to tolerated also suggest it wants to be bool.

Finally I don't think a host wide option will do. If it is to be of use
(considering that it used wrong may break applications), it needs
to be per-domain, and its value needs to be migrated (perhaps
in the form of a low/high pair of TSC frequency values).

Jan

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