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

[Andrew Jones]

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.

> 
> 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 chose ':' for the delimiter because ',' can't work, but
> if there's a better choice, then that's fine by me.)
> 
> Afaict a property list like this will require a new parser,
> which feels a bit funny since it seems we should already
> have support for this type of thing somewhere in QEMU. So,
> the question is: do we? I see we have array properties, but
> I don't believe that works with the command line. Should we
> only use QMP for this? We already want some QMP in order to
> query the supported vector lengths. Maybe we should use QMP
> to set the selection too? But then what about command line
> support for developers? And if the property is on the command
> line then we don't have to add it to the migration stream.
> 
> Thanks,
> drew