Re: [RFC 18/21] arm/cpu: Introduce a customizable kvm host cpu model

[Cornelia Huck <cohuck@redhat.com>]

On Mon, Oct 28 2024, Peter Maydell <peter.maydell@linaro.org> wrote:

> On Fri, 25 Oct 2024 at 14:24, Daniel P. Berrangé <berrange@redhat.com> wrote:
>> On Fri, Oct 25, 2024 at 03:18:25PM +0200, Eric Auger wrote:
>> > On 10/25/24 15:06, Daniel P. Berrangé wrote:
>> > > Also, is this naming convention really the same one that users
>> > > will see when they look at /proc/cpuinfo to view features ? It
>> > No it is not. I do agree that the custom cpu model is very low level. It
>> > is very well suited to test all series turning ID regs as writable but
>> > this would require an extra layer that adapts /proc/cpuinfo feature
>> > level to this regid/field abstraction.
>> >
>> > In /cpu/proc you will see somethink like:
>> >  Features    : fp asimd evtstrm aes pmull sha1 sha2 crc32 atomics fphp
>> > asimdhp cpuid asimdrdm lrcpc dcpop asimddp
>>
>> Right, IMHO, this is the terminology that QEMU must use in user
>> facing APIs.
>
> /proc/cpuinfo's naming is rather weird for historical
> reasons (for instance there is only one FEAT_FP16 feature
> but cpuinfo lists "fphp" and "asimdhp" separately).
> Currently QEMU only has to care about cpuinfo name tags
> in linux-user/elfload.c where there's a bunch of data
> structures for "what hwcaps do we need to advertise
> given what the CPU has?". I would definitely prefer it if
> we could use architectural feature names...
>
> On other architectures do we do anything to forbid
> invalid combinations? For Arm there are architectural
> rules about feature X requiring features Y and Z.
> Are we going to just let the user create a CPU that
> the guest OS will barf on if they want to? (The
> user-experience for that is potentially not very nice,
> because something like "-cpu cortex-a57,+sve" seems like
> something you might want to do but isn't actually valid;
> even listing the direct required dependency of FEAT_SVE
> like "-cpu cortex-a57,+sve,+fp16" isn't sufficient
> because SVE is optional-from-v8.2 and so a guest could
> in theory assume the presence of anything mandatory in
> v8.1 and v8.2. But even merely diagnosing invalid
> combinations is a huge pain, and automagically pulling
> in every mandatory implicit or explicit dependency
> seems like it might be surprising to users, as well as
> being annoying to implement. Currently we sidestep
> all of these problems by (a) only allowing the command
> line to disable a feature, not to enable it where it
> didn't previously exist and (b) mostly not providing
> command line flags for individual features...)

I think s390 does some dependency checking on features, and also rejects
features that are "too new".