On Fri, Apr 05, 2019 at 11:54:07AM +0200, Andrew Jones wrote:
> On Fri, Apr 05, 2019 at 10:35:55AM +0100, Dave Martin wrote:
> > On Thu, Apr 04, 2019 at 04:20:34PM +0200, Andrew Jones wrote:
> > > On Fri, Mar 29, 2019 at 01:00:45PM +0000, Dave Martin wrote:
> > > > KVM will need to interrogate the set of SVE vector lengths
> > > > available on the system.
> > > >
> > > > This patch exposes the relevant bits to the kernel, along with a
> > > > sve_vq_available() helper to check whether a particular vector
> > > > length is supported.
> > > >
> > > > __vq_to_bit() and __bit_to_vq() are not intended for use outside
> > > > these functions: now that these are exposed outside fpsimd.c, they
> > > > are prefixed with __ in order to provide an extra hint that they
> > > > are not intended for general-purpose use.
> > > >
> > > > Signed-off-by: Dave Martin <[email protected]>
> > > > Reviewed-by: Alex Bennée <[email protected]>
> > > > Tested-by: zhang.lei <[email protected]>
> > > > ---
> > > > arch/arm64/include/asm/fpsimd.h | 29 +++++++++++++++++++++++++++++
> > > > arch/arm64/kernel/fpsimd.c | 35
> > > > ++++++++---------------------------
> > > > 2 files changed, 37 insertions(+), 27 deletions(-)
> > > >
> > > > diff --git a/arch/arm64/include/asm/fpsimd.h
> > > > b/arch/arm64/include/asm/fpsimd.h
[...]
> > > > +/* Set of available vector lengths, as vq_to_bit(vq): */
> > >
> > > s/as/for use with/ ?
> >
> > Not exactly. Does the following work for you:
> >
> > /*
> > * Set of available vector lengths
> > * Vector length vq is encoded as bit __vq_to_bit(vq):
> > */
>
> Yes. That reads much better.
OK
> > > s/vq_to_bit/__vq_to_bit/
> >
> > Ack: that got renamed when I moved it to fpsimd.h, bit I clearly didn't
> > update the comment as I pasted it across.
> >
> > >
> > > > +extern __ro_after_init DECLARE_BITMAP(sve_vq_map, SVE_VQ_MAX);
> > > > +
> > > > +/*
> > > > + * Helpers to translate bit indices in sve_vq_map to VQ values (and
> > > > + * vice versa). This allows find_next_bit() to be used to find the
> > > > + * _maximum_ VQ not exceeding a certain value.
> > > > + */
> > > > +static inline unsigned int __vq_to_bit(unsigned int vq)
> > > > +{
> > >
> > > Why not have the same WARN_ON and clamping here as we do
> > > in __bit_to_vq. Here a vq > SVE_VQ_MAX will wrap around
> > > to a super high bit.
> > >
> > > > + return SVE_VQ_MAX - vq;
> > > > +}
> > > > +
> > > > +static inline unsigned int __bit_to_vq(unsigned int bit)
> > > > +{
> > > > + if (WARN_ON(bit >= SVE_VQ_MAX))
> > > > + bit = SVE_VQ_MAX - 1;
> > > > +
> > > > + return SVE_VQ_MAX - bit;
> > > > +}
> > > > +
> > > > +/* Ensure vq >= SVE_VQ_MIN && vq <= SVE_VQ_MAX before calling this
> > > > function */
> > >
> > > Are we avoiding putting these tests and WARN_ONs in this function to
> > > keep it fast?
> >
> > These are intended as backend for use only by fpsimd.c and this header,
> > so peppering them with WARN_ON() felt excessive. I don't expect a lot
> > of new calls to these (or any, probably).
> >
> > I don't recall why I kept the WARN_ON() just in __bit_to_vq(), except
> > that the way that gets called is a bit more complex in some places.
> >
> > Are you happy to replace these with comments? e.g.:
> >
> > /* Only valid when vq >= SVE_VQ_MIN && vq <= SVE_VQ_MAX */
> > __vq_to_bit()
> >
> > /* Only valid when bit < SVE_VQ_MAX */
> > __bit_to_vq()
> >
> >
> > OTOH, these are not used on fast paths, so maybe having both as
> > WARN_ON() would be better. Part of the problem is knowing what to clamp
> > to: these are generally used in conjunction with looping or bitmap find
> > operations, so the caller may be making assumptions about the return
> > value that may wrong when the value is clamped.
> >
> > Alternatively, these could be BUG() -- but that seems heavy.
> >
> > What do you think?
>
> I like the idea of having WARN_ON's to enforce the constraints. I
> wouldn't be completely opposed to not having anything other than
> the comments, though, as there is a limit to how defensive we should
> be. I'll abstain from this vote.
I'll have a think about whether there's anything non-toxic that we can
return in the error cases. If not, I may demote these to comments:
returning an actual error code for this sort of things feels like a
step too far.
Otherwise we can have WARNs.
Cheers
---Dave
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