Re: [PATCH] arm64/sme: Add hwcap for Scalable Matrix Extension

[Tianjia Zhang]

Hi Mark,

On 4/14/22 8:02 PM, Mark Brown wrote:
> On Thu, Apr 14, 2022 at 07:55:44PM +0800, Tianjia Zhang wrote:
>
> > Allow userspace to detect support for SME (Scalable Matrix Extension)
> > by providing a hwcap for it, using the official feature name FEAT_SME,
> > declared in ARM DDI 0487H.a specification.
>
> There's already a hwcap for the core feature and all the subfeatures
> added as part of the series I've been posting for SME:
>
>     https://lore.kernel.org/linux-arm-kernel/20220408114328.1401034-1-broonie@xxxxxxxxxx/
>
> Why add something independently, especially given that there is no way
> for userspace to do anything constructive with the feature without the
> rest of the kernel support?  Any attempt to use SME instructions without
> kernel support will trap and generate a SIGILL even if the feature is
> present in hardware.

Great job, I encountered the issue of invalid REVD (requires FEAT_SME)
instruction when developing SVE2 programs, so I plan to gradually
support SME in the kernel, thanks for your contribution, you can ignore
my patch.

In addition, I would like to ask a question, whether there is an
alternative SVE2 instruction for the REVD instruction that can complete
this operation, if the machine does not support SME.

> Do you have a system with SME that you're trying to use?  Review/testing
> on the current series would be appreciated.

Unfortunately, the value currently read by my machine ID_AA64PFR1_EL1
register is 0x121. It seems that the hardware does not support SME. Is
there any other help I can provide?

Kind regards,
Tianjia