linux-kernel - Re: [RFC PATCH 1/2] arm64/tracing: add cntvct based trace clock

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Date:   Wed, 24 Nov 2021 10:45:44 +0100
From:   Nicolas Saenz Julienne <nsaenzju@...hat.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     linux-arm-kernel@...ts.infradead.org, maz@...nel.org,
        james.morse@....com, alexandru.elisei@....com,
        suzuki.poulose@....com, catalin.marinas@....com, will@...nel.org,
        linux-kernel@...r.kernel.org, kvmarm@...ts.cs.columbia.edu,
        mingo@...hat.com, mtosatti@...hat.com, nilal@...hat.com,
        Linux Trace Devel <linux-trace-devel@...r.kernel.org>
Subject: Re: [RFC PATCH 1/2] arm64/tracing: add cntvct based trace clock

On Mon, 2021-11-22 at 09:57 -0500, Steven Rostedt wrote:
> On Fri, 19 Nov 2021 11:21:17 +0100
> Nicolas Saenz Julienne <nsaenzju@...hat.com> wrote:
> 
> > +/* SPDX-License-Identifier: GPL-2.0 */
> > +#ifndef _ASM_ARM64_TRACE_CLOCK_H
> > +#define _ASM_ARM64_TRACE_CLOCK_H
> > +
> > +#include <linux/types.h>
> > +
> > +extern u64 notrace trace_clock_arm64_cntvct(void);
> > +
> > +# define ARCH_TRACE_CLOCKS \
> > +	{ trace_clock_arm64_cntvct, "cntvct", .in_ns = 0 },
> > +
> > +#endif  /* _ASM_ARM64_TRACE_CLOCK_H */
> 
> So this will appear as a usable clock in trace-cmd.
> 
> And since this will be used to synchronize between host and guest like the
> x86_tsc is used, that means that trace-cmd needs to know that this is the
> an arch "CPU" clock. I wonder if we should rename x86_clock (or at least
> make it an alias) to "kvm_clock". Then we can have trace-cmd use
> "kvm_clock" as the clock for synchronization between host and guests for
> all architectures?
>
> Thinking about this, instead of renaming it, I'll add code to create an
> alias to these clocks. Then every arch can pick what clock is used that is
> the same between hosts and guests such that user space tooling doesn't have
> to keep a database of what clocks are used for synchronization between
> hosts and guests for each arch.
> 
> I'll go add some code ;-)

I really like the idea, please keep me in the loop if you send something
upstream.

-- 
Nicolás Sáenz