Re: [Qemu-devel] How do we do user input bitmap properties?

[Andrew Jones]

On Thu, Apr 18, 2019 at 11:52:04AM +0100, Dave Martin wrote:
> On Thu, Apr 18, 2019 at 10:46:34AM +0100, Andrew Jones wrote:
> > On Thu, Apr 18, 2019 at 11:28:41AM +0200, Andrew Jones wrote:
> > > Hi all,
> > > 
> > > First some background:
> > > 
> > > For the userspace side of AArch64 guest SVE support we need to
> > > expose KVM's allowed vector lengths bitmap to the user and allow
> > > the user to choose a subset of that bitmap. Since bitmaps are a
> > > bit awkward to work with then we'll likely want to expose it as
> > > an array of vector lengths instead. Also, assuming we want to
> > > expose the lengths as number-of-quadwords (quadword == 128 bits
> > > for AArch64 and vector lengths must be multiples of quadwords)
> > > rather than number-of-bits, then an example array (which will
> > > always be a sequence) might be
> > > 
> > >  [ 8, 16, 32 ]
> > > 
> > > The user may choose a subsequence, but only through truncation,
> > > i.e. [ 8, 32 ] is not valid, but [ 8, 16 ] is.
> > > 
> > > Furthermore, different hosts may support different sequences
> > > which have the same maximum. For example, if the above sequence
> > > is for Host_A, then Host_B could be
> > > 
> > >  [ 8, 16, 24, 32 ]
> > 
> > It doesn't really matter for this discussion, but I just realized
> > that I picked bogus numbers for these examples. 32 would be too
> > big. The largest supported is 16. I probably should have just used
> > the simple [ 1, 2, 4 ] and [ 1, 2, 3, 4 ] arrays, corresponding to
> > vector lengths (in bits) 128, 256, 512 and 128, 256, 384, 512.
> 
> I'd argue differently from the ABI perspective.
> 
> If the host supports vq = [ 1,2,3,4 ], then it is entirely valid to ask
> for [ 1,2,4 ].  KVM will fail the KVM_REG_ARM64_SVE_VLS write with
> EINVAL, but I don't distinguish between a set that is not satisfiable on
> this hardware and a set that is not valid at all.  The kernel basically
> doesn't check for the latter: an architecturally invalid set will always
> not be satisfiable on the hardware.

I agree that 1,2,4 should be a valid set, but how do we avoid the user
attempting to select it on a platform that supports 1,2,3,4 now? Until
the hardware and KVM will allow it, then I don't think we should try
to "support" it in any way. Maybe we should name the property such that
it's clear we need to use truncation when constructing a subset now.
This would allow us to add another, more generally named, property later
when we can select nearly arbitrary subsets.

> 
> Ideally, userspace should not preempt this decision, in case the
> architecture becomes more flexible someday in what it can virtualise.
> 
> > > The host must support all lengths in the sequence, which means
> > > that while Host_A supports 32, since it doesn't support 24 and
> > > we can only truncate sequences, we must use either [ 8 ] or
> > > [ 8, 16 ] for a compatible sequence if we intend to migrate
> > > between the hosts.
> > > 
> > > Now to the $SUBJECT question:
> > > 
> > > My feeling is that we should require the sequence to be
> > > provided on the command line as a cpu property. Something
> > > like
> > > 
> > >   -cpu host,sve-vl-list=8:16
> 
> I can't decide whether it's more or less user-friendly to denote VL in
> terms of bytes in this kind of context.

I'm not sure either. Switching to my second take at quadwords this should
have been '1:2'. In bytes [ 1, 2 ] would be '16:32'.

> 
> Usually when I say "VL" in the kernel ABI I try to mean this.
> (KVM_REG_ARM64_SVE_VLS is a special case: here the lengths are fancily
> encoded rather than being passed as integers, so the precise unit used
> is a more abstract concept here ... or anyway, that's my excuse.
> _SVE_VQS felt odd here.)

My thinking is that using the number of quadwords keeps the numbers
smaller. But, if people are mostly going to want to think about vector
lengths in terms of bits, like the specification does, then maybe we
should just let the numbers be bigger: [ 128, 256, 512 ]

> 
> For kvmtool I went with quadwords on the command line, but only as a
> quick hack to simplify the parser slightly.
> 
> OTOH, specifying 1,2,4 on the command line is clearly less annoying and
> error-prone than 16,32,64.
> 
> Naming the option with "vq" instead of "vl" is another option, though
> "vq" is Linux terminology not endorsed by the architecture.

I think VL is OK as long we document the unit.

> 
> For kvmtool I've considered a range / filtering syntax, but I didn't
> implement it yet.

I think we want each VL to be explicitly listed on the command line
for the compatibility issue described above. On one host the range
[1,4] can be different than the range [1,4] on anther host, depending
on whether '3' is in there. And syntax like 1-4:-3 would be overly
complicated, IMO. So I'm inclined to just require all of them to be
listed, but of course order wouldn't matter.

Thanks,
drew