On Mon, Jun 24, 2019 at 01:49:11PM +0200, Andrew Jones wrote: > On Mon, Jun 24, 2019 at 12:05:26PM +0100, Dave Martin wrote: > > On Fri, Jun 21, 2019 at 05:34:15PM +0100, Andrew Jones wrote: > > > Introduce cpu properties to give fine control over SVE vector lengths. > > > We introduce a property for each valid length up to the current > > > maximum supported, which is 2048-bits. The properties are named, e.g. > > > sve128, sve256, sve512, ..., where the number is the number of bits. > > > > > > It's now possible to do something like -cpu max,sve-max-vq=4,sve384=off > > > to provide a guest vector lengths 128, 256, and 512 bits. The resulting > > > set must conform to the architectural constraint of having all power-of-2 > > > lengths smaller than the maximum length present. It's also possible to > > > only provide sve<vl-bits> properties, e.g. -cpu max,sve512=on. That > > > example provides the machine with 128, 256, and 512 bit vector lengths. > > > It doesn't hurt to explicitly ask for all expected vector lengths, > > > which is what, for example, libvirt should do. > > > > > > Note1, it is not possible to use sve<vl-bits> properties before > > > sve-max-vq, e.g. -cpu max,sve384=off,sve-max-vq=4, as supporting > > > that overly complicates the user input validation. > > > > > > Note2, while one might expect -cpu max,sve-max-vq=4,sve512=on to be the > > > same as -cpu max,sve512=on, they are not. The former enables all vector > > > lengths 512 bits and smaller, while the latter only sets the 512-bit > > > length and its smaller power-of-2 lengths. It's probably best not to use > > > sve-max-vq with sve<vl-bits> properties, but it can't be completely > > > forbidden as we want qmp_query_cpu_model_expansion to work with guests > > > launched with e.g. -cpu max,sve-max-vq=8 on their command line. > > > > Supporting both options together in a non-idempotent way seems to > > complicate things. > > > > Would it be simpler to allow sve-max-vq only when there are no sve<bits> > > options? > > Not really. Since we can't simply remove sve-max-vq from the 'max' cpu > type, then we'd still need conditions to check when we can use it. The > restriction that it has to come first reduces the complexity > substantially, and I think restricting to not being allowed > at all, when sve<vl-bits> are used, would only give us a net check > reduction of one or two. > > > > > Alternatively, the two options would be defined so that their meanings > > are independent of parse order. > > > > So, way sve-max-vq=<max> means that: > > > > * all VQs up to max for which there is no corresponding sve<bits>=off, > > are enabled; and > > > > * VQ max is enabled; and > > > > * all VQs exceeding max are disabled. > > > > While sve<bits>=(on|off) means > > > > * the VQ coresponding to <bits> is enabled (for on) or disabled (for > > off). > > > > After parsing all the options, you check that the sve-max-vq and > > sve<bits> optinos are consistent. If you disallow duplicate sve-max-vq > > or sve<bits> options, then there is no possibility of ambiguity and the > > order or options doesn't matter. > > I don't want to put any final checks at the end of parsing all options, > because that won't work with the QMP query.
Actually, I think I can allow sve-max-vq to come after sve<vl-bits> without adding much code, and without requiring a final check. I could try that if you'd like, but I'm not sure it's worth it. Also, I feel like I tried this once already and rejected it for some reason, but atm I can't remember why. > > > > > (There may be constraints on the way qemu options parsing works that > > make this hard, though...) > > I can't think of any issue with the parsing, but the QMP query only using > the property get accessors, without any finalizing, does put constraints > on what we can do. > > > > > Having sve-max-vq in 128-bit units while sve<bits> are named in terms of > > bit counts also feels a bit odd now. > > sve-max-vq already exists, so it'd have to be deprecated if we don't want > it anymore. And I think sve<vl-bits> is the right naming to go with for > that. Of course using sve-max-vq is completely optional. If you don't want > it for backward compatible reasons, or as a shorthand to restrict the > lengths, then you can just use the sve<vl-bits> properties. Indeed, with > KVM, if you use the 'host' cpu type (the default for use with KVM), then > you won't even have the sve-max-vq property. As 'host' never had it, I > didn't have to keep it, nor adopt it. And of course I didn't want to > adopt it. > > Thanks, > drew >
