Re: [PATCH v3 01/10] arm64: pmu: Add hook to handle pmu-related undefined instructions

[Will Deacon]

On Tue, Sep 29, 2020 at 08:46:46AM -0500, Rob Herring wrote:
> On Mon, Sep 28, 2020 at 1:26 PM Will Deacon <will@xxxxxxxxxx> wrote:
> > On Fri, Sep 11, 2020 at 03:51:09PM -0600, Rob Herring wrote:
> > > +static int emulate_pmu(struct pt_regs *regs, u32 insn)
> > > +{
> > > +     u32 rt;
> > > +     u32 pmuserenr;
> > > +
> > > +     rt = aarch64_insn_decode_register(AARCH64_INSN_REGTYPE_RT, insn);
> > > +     pmuserenr = read_sysreg(pmuserenr_el0);
> > > +
> > > +     if ((pmuserenr & (ARMV8_PMU_USERENR_ER|ARMV8_PMU_USERENR_CR)) !=
> > > +         (ARMV8_PMU_USERENR_ER|ARMV8_PMU_USERENR_CR))
> > > +             return -EINVAL;
> > > +
> > > +
> > > +     /*
> > > +      * Userspace is expected to only use this in the context of the scheme
> > > +      * described in the struct perf_event_mmap_page comments.
> > > +      *
> > > +      * Given that context, we can only get here if we got migrated between
> > > +      * getting the register index and doing the MSR read.  This in turn
> > > +      * implies we'll fail the sequence and retry, so any value returned is
> > > +      * 'good', all we need is to be non-fatal.
> > > +      *
> > > +      * The choice of the value 0 is comming from the fact that when
> > > +      * accessing a register which is not counting events but is accessible,
> > > +      * we get 0.
> > > +      */
> > > +     pt_regs_write_reg(regs, rt, 0);
> >
> > Hmm... this feels pretty fragile since, although we may expect userspace only
> > to trigger this in the context of the specific perf use-case, we don't have
> > a way to detect that, so the ABI we're exposing is that EL0 accesses to
> > non-existent counters will return 0. I don't really think that's something
> > we want to commit to.
> >
> > When restartable sequences were added to the kernel, one of the proposed
> > use-cases was to allow PMU access on big/little systems, because the
> > sequence will abort on preemption. Taking that approach removes the need
> > for this emulation hook entirely. Is that something we can rely on instead
> > of this emulation hook?
> 
> So back to the RFC version[1]!? That would mean pulling librseq into
> the kernel based on the prior discussion. It doesn't look like that
> has happened yet.

Yeah, or just don't bother supporting heterogeneous systems with this
for now.

> Why not just drop the undef hook? For heterogeneous systems, we
> require userspace to pin itself to cores for a specific PMU. See patch
> 9. If userspace fails to do that, then it gets to keep the pieces.

Dropping it works too!

Will