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

[Andrea Bolognani <abologna@redhat.com>]

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.

-- 
Andrea Bolognani / Red Hat / Virtualization