Re: [Qemu-arm] [Qemu-devel] [PATCH v2 05/14] target/arm/helper: zcr: Add build bug next to value range assumption

[Andrew Jones]

On Tue, Jun 25, 2019 at 08:11:27AM +0200, Andrew Jones wrote:
> On Mon, Jun 24, 2019 at 05:03:08PM +0100, Dave Martin wrote:
> > On Mon, Jun 24, 2019 at 12:30:37PM +0100, Andrew Jones wrote:
> > > On Mon, Jun 24, 2019 at 12:05:07PM +0100, Dave Martin wrote:
> > > > On Fri, Jun 21, 2019 at 05:34:13PM +0100, Andrew Jones wrote:
> > > > 
> > > > The purpose of this check should probably at least be described in a
> > > > comment -- i.e., what actually depends on this?
> > > 
> > > I was thinking the already present "Bits other than [3:0] are RAZ/WI."
> > > explained that, but how about this for an improvement?
> > > 
> > > /*
> > >  * Only the lowest 4 bits of ZCR_ELx may be used to constrain the vector
> > >  * length, the rest of the bits are RAZ/WI. Since the vector length of
> > >  * 128-bits (1 in quadwords) is represented as zero in ZCR_ELx, and all
> > >  * vector lengths are represented as their length in quadwords minus 1,
> > >  * then four bits allow up to quadword 16 to be selected.
> > >  */
> > 
> > No, maybe the existing comment is enough.
> > 
> > I thought there might be more code elsewhere that assumes that checks
> > sve_max_vq <= ARM_MAX_VQ then then assumes that sve_max_vq <= 16.  But
> > if not, we probably don't need an additional comment here.
> 
> I suppose there is some assumption that if sve_max_vq > 0 then it is
> also <= ARM_MAX_VQ elsewhere in QEMU. However here in zcr_write I don't
> think that assumption is being used. Here we're simply enforcing a limit
> of 16 within the emulation, without checking sve_max_vq at all. So I like
> the suggestion for a build bug like the one this patch adds, because
> otherwise we have 16 in two separate places; the ARM_MAX_VQ definition
> and the '& 0xf'.

I suppose we could also write the 0xf in terms of ARM_MAX_VQ, with a
(ARM_MAX_VQ - 1), but that's getting into emulation implementation
preferences, which I don't know anything about. So I'd leave that to
Richard and Peter.

> 
> > 
> > I haven't tried to understand all the code in the series beyond the
> > user/kernel interactions, so maybe I was just paranoid.
> 
> Paranoia is good for the soul. Or something like that...
> 
> Thanks,
> drew