Re: [Qemu-devel] [PATCH 00/13] target/arm/kvm: enable SVE in guests

[Dave Martin]

On Wed, May 15, 2019 at 12:28:20PM +0100, Andrea Bolognani wrote:
> On Wed, 2019-05-15 at 12:14 +0100, Dave Martin wrote:
> > On Wed, May 15, 2019 at 09:03:58AM +0100, Andrea Bolognani wrote:
> > > On Tue, 2019-05-14 at 13:14 -0700, Richard Henderson wrote:
> > > > Why is =4 less user-friendly than =512?
> > > > 
> > > > I don't actually see "total bits in vector" as more user-friendly than 
> > > > "number
> > > > of quadwords" when it comes to non-powers-of-2 like =7 vs =896 or =13 
> > > > vs =1664.
> > > 
> > > I would wager most people are intimately familiar with bits, bytes
> > > and multiples due to having to work with them daily. Quadwords, not
> > > so much.
> > 
> > Generally I tend to agree.  For kvmtool I leaned torward quadwords
> > purely because
> > 
> >     16,32,48,64,80,96,112,128,144,160,176,192,208
> > 
> > is a big pain to type compared with
> > 
> >     1,2,3,4,5,6,7,8,9,10,11,12,13
> > 
> > Even though I prefer to specify vector lengths in bytes everywhere else
> > in the Linux user API (precisely to avoid the confusion you object to).
> > 
> > This isn't finalised yet for kvmtool -- I need to rework the patches
> > and may not include it at all initially: kvmtool doesn't support
> > migration, which is the main usecase for being able to specify an exact
> > set of vector lengths AFAICT.
> > 
> > Since this is otherwise only useful for migration, experimentation or
> > machine-driven configuration, a bitmask
> > 
> >     0x1fff
> > 
> > as some have suggested may well be a pragmatic alternative for kvmtool.
> 
> Just to be clear, I have suggested using bits (or bytes or megabytes
> depending on the exact value) only for the command-line-user-oriented
> sve-vl-max option, which would take a single value.
> 
> For interoperation with the management layer, on the other hand,
> using a bitmap is perfectly fine, and whether the values encoded
> within are expressed in quadwords or whatever other format is largely
> irrelevant, so long as it it's properly documented of course.

Seems fair.

Cheers
---Dave