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

[Tianjia Zhang]

Hi Mark,

On 4/19/22 9:58 PM, Mark Brown wrote:
> On Fri, Apr 15, 2022 at 10:25:33AM +0800, Tianjia Zhang wrote:
> > On 4/14/22 8:02 PM, Mark Brown wrote:
> > > On Thu, Apr 14, 2022 at 07:55:44PM +0800, Tianjia Zhang wrote:
>
> > > 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.
>
> I see.  Unfortunately all the new registers mean that we really need to
> define all the ABI as soon as we enable anything and the only thing we
> can really skip out on when doing initial enablement is KVM (which I
> have in fact skipped for the time being, I'll look at that at some point
> after the initial support is landed).
>
> > 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.
>
> I'm not aware of anything, but I am mostly focused on the OS support
> rather than any of the actual mathematical operations that are more the
> point of these architecture features so I might be missing something.
>
> > > 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?
>
> Other than verifying that the series doesn't cause trouble for systems
> without SME

Thanks for your reply, I have indirectly implemented the functionality
of the REVD instruction using the tbl instruction on a machine that does
not support SME.

For this group of patchsets, I will do some tests later, which may take
a long time, and there is currently no exclusive machine at hand.

Best regards,
Tianjia