Re: [PATCH v11 10/40] arm64/sme: Basic enumeration support

[Szabolcs Nagy <szabolcs.nagy@arm.com>]

The 02/21/2022 14:32, Catalin Marinas wrote:
> On Mon, Feb 07, 2022 at 03:20:39PM +0000, Mark Brown wrote:
> > diff --git a/Documentation/arm64/elf_hwcaps.rst b/Documentation/arm64/elf_hwcaps.rst
> > index b72ff17d600a..5626cf208000 100644
> > --- a/Documentation/arm64/elf_hwcaps.rst
> > +++ b/Documentation/arm64/elf_hwcaps.rst
> > @@ -259,6 +259,39 @@ HWCAP2_RPRES
> >  
> >      Functionality implied by ID_AA64ISAR2_EL1.RPRES == 0b0001.
> >  
> > +HWCAP2_SME
> > +
> > +    Functionality implied by ID_AA64PFR1_EL1.SME == 0b0001, as described
> > +    by Documentation/arm64/sme.rst.
> > +
> > +HWCAP2_SME_I16I64
> > +
> > +    Functionality implied by ID_AA64SMFR0_EL1.I16I64 == 0b1111.
> > +
> > +HWCAP2_SME_F64F64
> > +
> > +    Functionality implied by ID_AA64SMFR0_EL1.F64F64 == 0b1.
> > +
> > +HWCAP2_SME_I8I32
> > +
> > +    Functionality implied by ID_AA64SMFR0_EL1.I8I32 == 0b1111.
> > +
> > +HWCAP2_SME_F16F32
> > +
> > +    Functionality implied by ID_AA64SMFR0_EL1.F16F32 == 0b1.
> > +
> > +HWCAP2_SME_B16F32
> > +
> > +    Functionality implied by ID_AA64SMFR0_EL1.B16F32 == 0b1.
> > +
> > +HWCAP2_SME_F32F32
> > +
> > +    Functionality implied by ID_AA64SMFR0_EL1.F32F32 == 0b1.
> > +
> > +HWCAP2_SME_FA64
> > +
> > +    Functionality implied by ID_AA64SMFR0_EL1.FA64 == 0b1.
> 
> More of a question for the libc people: should we drop the fine-grained
> HWCAP corresponding to the new ID_AA64SMFR0_EL1 register (only keep
> HWCAP2_SME) and get the user space to use the MRS emulation? Would any
> ifunc resolver be affected?
> 

good question.

within glibc HWCAP2_SME is enough (to decide if we need to
deal with additional register state and the lazy ZA save
scheme) but i guess user code that actually uses sme would
need the details (including in ifunc resolvers in principle).

since we have mrs, there is no strict need for hwcaps.
if ifunc resolvers using this info are not widespread then
the mrs emulation overhead is acceptable, but i suspect
hwcaps are nicer to use.

do we have a plan after hwcap2 bits run out? :)