Re: [PATCH 13/13] arm64: Add CNT{P,V}CTSS_EL0 alternatives to cnt{p,v}ct_el0

From: Marc Zyngier <maz@kernel.org>
To: Oliver Upton <oupton@google.com>
Cc: linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org, Mark Rutland <mark.rutland@arm.com>,
	Daniel Lezcano <daniel.lezcano@linaro.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Peter Shier <pshier@google.com>,
	Raghavendra Rao Ananta <rananta@google.com>,
	Ricardo Koller <ricarkol@google.com>,
	Will Deacon <will@kernel.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Linus Walleij <linus.walleij@linaro.org>,
	kernel-team@android.com
Subject: Re: [PATCH 13/13] arm64: Add CNT{P,V}CTSS_EL0 alternatives to cnt{p,v}ct_el0
Date: Mon, 09 Aug 2021 19:11:17 +0100	[thread overview]
Message-ID: <87im0ebi9m.wl-maz@kernel.org> (raw)
In-Reply-To: <CAOQ_QshjRWh=onT-j3dWgmVfnAXsMFJoz0i7OEezQxkW4O9KZA@mail.gmail.com>

On Mon, 09 Aug 2021 17:42:00 +0100,
Oliver Upton <oupton@google.com> wrote:
> 
> On Mon, Aug 9, 2021 at 8:48 AM Marc Zyngier <maz@kernel.org> wrote:
> >
> > CNTPCTSS_EL0 and CNTVCTSS_EL0 are alternatives to the usual
> > CNTPCT_EL0 and CNTVCT_EL0 that do not require a previous ISB
> > to be synchronised (SS stands for Self-Synchronising).
> >
> > Use the ARM64_HAS_ECV capability to control alternative sequences
> > that switch to these low(er)-cost primitives. Note that the
> > counter access in the VDSO is for now left alone until we decide
> > whether we want to allow this.
> 
> What remains to be figured out before we add this to the vDSO (and
> presumably advertise to userspace through some standard convention)?

We need to understand what breaks if we runtime-patch the VDSO just
like we do with the rest of the kernel. To start with, the debug
version of the shared object is not the same as the object presented
to the process. Maybe that's not a problem, but I would tend to err on
the side of caution.

An alternative suggested by Ard was to have a separate function
altogether for the counter access and an ifunc mapping to pick the
right one.

> It would be nice to skip the trap handler altogether, unless there's a
> can of worms lurking that I'm not aware of.

The trap handlers are only there to work around errata. If you look at
the arch timer code, you will notice that there is a bunch of SoCs and
CPUs that do not have a reliable counter, and for which we have to
trap the virtual counter accesses from userspace (as well as the
VDSO).

On sane platforms, userspace is free to use the virtual counter
without any trap.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.