Re: [PATCH v7 07/12] xen: enable Dom0 to use SVE feature

[Luca Fancellu <Luca.Fancellu@xxxxxxx>]

> On 23 May 2023, at 11:31, Jan Beulich <jbeulich@xxxxxxxx> wrote:
> 
> On 23.05.2023 12:21, Luca Fancellu wrote:
>>> On 23 May 2023, at 11:02, Jan Beulich <jbeulich@xxxxxxxx> wrote:
>>> On 23.05.2023 09:43, Luca Fancellu wrote:
>>>> @@ -838,6 +838,22 @@ Controls for how dom0 is constructed on x86 systems.
>>>> 
>>>>    If using this option is necessary to fix an issue, please report a bug.
>>>> 
>>>> +Enables features on dom0 on Arm systems.
>>>> +
>>>> +*   The `sve` integer parameter enables Arm SVE usage for Dom0 domain and 
>>>> sets
>>>> +    the maximum SVE vector length, the option is applicable only to 
>>>> AArch64
>>>> +    guests.
>>> 
>>> Why "guests"? Does the option affect more than Dom0?
>> 
>> I used “guests” because in my mind I was referring to all the aarch64 OS 
>> that can be used
>> as control domain, I can change it if it sounds bad.
> 
> If you means OSes then better also say OSes. But maybe this doesn't need
> specifically expressing, by saying e.g. "..., the option is applicable
> only on AArch64"? Or can a Dom0 be 32-bit on Arm64 Xen?

I think there is no limitation so Dom0 can be 32 bit or 64. Maybe I can say
“... AArch64 kernel guests.”?

> 
>>>> +    A value equal to 0 disables the feature, this is the default value.
>>>> +    Values below 0 means the feature uses the maximum SVE vector length
>>>> +    supported by hardware, if SVE is supported.
>>>> +    Values above 0 explicitly set the maximum SVE vector length for Dom0,
>>>> +    allowed values are from 128 to maximum 2048, being multiple of 128.
>>>> +    Please note that when the user explicitly specifies the value, if 
>>>> that value
>>>> +    is above the hardware supported maximum SVE vector length, the domain
>>>> +    creation will fail and the system will stop, the same will occur if 
>>>> the
>>>> +    option is provided with a non zero value, but the platform doesn't 
>>>> support
>>>> +    SVE.
>>> 
>>> Assuming this also covers the -1 case, I wonder if that isn't a little too
>>> strict. "Maximum supported" imo can very well be 0.
>> 
>> Maximum supported, when platforms uses SVE, can be at minimum 128 by arm 
>> specs.
> 
> When there is SVE - sure. But when there's no SVE, 0 is kind of the implied
> length. And I'd view a command line option value of -1 quite okay in that
> case: They've asked for the maximum supported, so they'll get 0. No reason
> to crash the system during boot.

Ok I see what you mean, for example when Kconfig SVE is enabled, but the 
platform doesn’t
have SVE feature, requesting sve=-1 will keep the value to 0, and no system 
will be stopped.

Maybe I can say: 

“... the same will occur if the option is provided with a positive non zero 
value,
but the platform doesn't support SVE."



> 
> Jan