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

[Marc Zyngier]

On Mon, 09 Aug 2021 19:17:38 +0100,
Oliver Upton <oupton@xxxxxxxxxx> wrote:
> 
> 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.

Urghhh... This reminds me of

https://git.kernel.org/pub/scm/linux/kernel/git/maz/arm-platforms.git/commit/?h=arm/netwinder&id=72797818a31d37a7ec28db659afcab0a56d47968

which I never tried to get merged for this exact reason. I'd rather
not replicate this sort of braindamage^Wthing if I can avoid it.

Thanks,

	M.

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