Re: [RFC] Adding the A64FX's HPC funtions.

[Peter Maydell]

On Wed, 2 Jun 2021 at 20:02, Richard Henderson
<richard.henderson@linaro.org> wrote:
> On 6/1/21 8:21 AM, Peter Maydell wrote:
> >>> 2) Is it OK to specify the option to set the HPC extension of A64FX as 
> >>> follows,
> >>> for example?
> >>>
> >>> -M virt -cpu max,a64fx-hpc-sec=on (*sector cache function) -M virt -cpu
> >>> max,a64fx-hpc-hwpf=on (*hardware prefetvh assist function) -M virt -cpu
> >>> max,a64fx-hpc-hwb=on (*hardware barrier function)
> >>>
> >>> It is also possible to implement something like -cpu a64fx, but since we 
> >>> don't
> >>> know if we can implement it immediately, we assume that we will use the 
> >>> -cpu
> >>> max option first.
>
> My first thought is that -cpu max can simply enable the extensions, without
> extra flags.  The max cpu has all of the features that we can enable, and as I
> see it this is just one more.

I dunno, because it's not an architectural feature, it's an implementation
feature. We can rely on architectural features not to step on each others'
toes, but there's no guarantee that some other impdef feature might not
clash with these a64fx ones.

Also, how does the guest OS typically detect the presence of these
features? If it does it by looking for MIDR etc values that say
"this is an A64FX" then -cpu max won't trigger that.

> I would like to add -cpu a64fx at some point.  But as you say, that need not
> happen right away.

I think it would be worth scoping out how much work the a64fx CPU
would require (ie what else does it need beyond these extensions
and whatever features we currently implement?). If that's not a
lot of work it might be simpler to just add it (possibly even
add it but with one or two of its features as not-yet-implemented.)

thanks
-- PMM