Re: [PATCH 07/10] xen/physinfo: add arm SVE arch capability and vector length

[Jan Beulich <jbeulich@xxxxxxxx>]

On 10.02.2023 16:54, Luca Fancellu wrote:
>> On 2 Feb 2023, at 12:05, Jan Beulich <jbeulich@xxxxxxxx> wrote:
>> On 02.02.2023 12:08, Luca Fancellu wrote:
>>> (is hw_cap only for x86?)
>>
>> I suppose it is, but I also expect it would better go away than be moved.
>> It doesn't hold a complete set of information, and it has been superseded.
>>
>> Question is (and I think I did raise this before, perhaps in different
>> context) whether Arm wouldn't want to follow x86 in how hardware as well
>> as hypervisor derived / used ones are exposed to the tool stack
>> (XEN_SYSCTL_get_cpu_featureset). I guess there's nothing really precluding
>> that data to consist of more than just boolean fields.
> 
> Yes I guess that infrastructure could work, however I don’t have the 
> bandwidth to
> put it in place at the moment, so I would like the Arm maintainers to give me 
> a
> suggestion on how I can expose the vector length to XL if putting its value 
> here
> needs to be avoided

Since you've got a reply from Andrew boiling down to the same suggestion
(or should I even say request), I guess it wants seriously considering
to introduce abstract base infrastructure first. As Andrew says, time not
invested now will very likely mean yet more time to be invested later.

>>> --- a/xen/include/public/sysctl.h
>>> +++ b/xen/include/public/sysctl.h
>>> @@ -18,7 +18,7 @@
>>> #include "domctl.h"
>>> #include "physdev.h"
>>>
>>> -#define XEN_SYSCTL_INTERFACE_VERSION 0x00000015
>>> +#define XEN_SYSCTL_INTERFACE_VERSION 0x00000016
>>
>> Why? You ...
>>
>>> @@ -104,7 +110,8 @@ struct xen_sysctl_physinfo {
>>>     uint32_t cpu_khz;
>>>     uint32_t capabilities;/* XEN_SYSCTL_PHYSCAP_??? */
>>>     uint32_t arch_capabilities;/* XEN_SYSCTL_PHYSCAP_{X86,ARM,...}_??? */
>>> -    uint32_t pad;
>>> +    uint16_t arm_sve_vl_bits;
>>> +    uint16_t pad;
>>>     uint64_aligned_t total_pages;
>>>     uint64_aligned_t free_pages;
>>>     uint64_aligned_t scrub_pages;
>>
>> ... add no new fields, and the only producer of the data zero-fills the
>> struct first thing.
> 
> Yes that is right, I will wait to understand how I can proceed here:
> 
> 1) rename arch_capabilities to arm_sve_vl_bits and put vector length there.
> 2) leave arch_capabilities untouched, no flag creation/setting, create 
> uint32_t arm_sve_vl_bits field removing “pad”,
>     Use its value to determine if SVE is present or not.
> 3) ??

3) Introduce suitable #define(s) to use part of arch_capabilities for your
purpose, without renaming the field. (But that's of course only if Arm
maintainers agree with you on _not_ going the proper feature handling route
right away.)

Jan