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

[Oliver Upton]

On Mon, Aug 9, 2021 at 11:11 AM Marc Zyngier <maz@xxxxxxxxxx> wrote:
>
> On Mon, 09 Aug 2021 17:42:00 +0100,
> Oliver Upton <oupton@xxxxxxxxxx> wrote:
> >
> > On Mon, Aug 9, 2021 at 8:48 AM Marc Zyngier <maz@xxxxxxxxxx> 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.

I would too, but there sadly are instances of Linux patching *user*
memory already (go look at how KVM/x86 handles the VMCALL/VMMCALL
instruction). But yes, I would much prefer the debug vDSO correspond
to the actual instructions.

> 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.
>

Hmm, this does sound promising.

> > 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.

/facepalm I was about 2 cups of coffee short when writing this :) Thanks!

--
Oliver