Re: [PATCH v4 5/7] arm64: Add support for FEAT_{LS64, LS64_V}

[Jonathan Cameron <jonathan.cameron@huawei.com>]

On Thu, 11 Sep 2025 16:50:14 +0100
Will Deacon <will@kernel.org> wrote:

> On Tue, Sep 09, 2025 at 09:48:04AM +0800, Yicong Yang wrote:
> > On 2025/9/8 20:01, Will Deacon wrote:  
> > > On Tue, Jul 15, 2025 at 04:13:54PM +0800, Yicong Yang wrote:  
> > >> diff --git a/Documentation/arch/arm64/elf_hwcaps.rst b/Documentation/arch/arm64/elf_hwcaps.rst
> > >> index 69d7afe56853..9e6db258ff48 100644
> > >> --- a/Documentation/arch/arm64/elf_hwcaps.rst
> > >> +++ b/Documentation/arch/arm64/elf_hwcaps.rst
> > >> @@ -435,6 +435,12 @@ HWCAP2_SME_SF8DP4
> > >>  HWCAP2_POE
> > >>      Functionality implied by ID_AA64MMFR3_EL1.S1POE == 0b0001.
> > >>  
> > >> +HWCAP3_LS64
> > >> +    Functionality implied by ID_AA64ISAR1_EL1.LS64 == 0b0001.
> > >> +
> > >> +HWCAP3_LS64_V
> > >> +    Functionality implied by ID_AA64ISAR1_EL1.LS64 == 0b0010.  
> > > 
> > > Given that these instructions only work on IMPLEMENTATION DEFINED memory
> > > locations and aren't guaranteed to generate an abort if used elsewhere,
> > > how is userspace supposed to know what to do with them?
> > >   
> > 
> > per ARM DDI0487 L.b section C3.2.6,
> > 
> >   When the instructions access a memory type that is not one of the following,
> >   a data abort for unsupported Exclusive or atomic access is generated...  
> 
> That's about the memory _type_. I'm talking about a supported memory type
> (e.g. writeback cacheable) but when the physical location doesn't support
> the instruction. That's captured a little later in the same section:
> 
>   | If the target memory location does not support the LD64B or ST64B
>   | instructions, then one of the following behaviors occurs:
>   |  * A stage 1 Data Abort, reported using the DFSC code of 0b110101,
>   |    is generated.
>   |  * The instruction performs the memory accesses, but the accesses
>   |    are not single-copy atomic above the byte level
> 
> and I think that's a bad interface to expose blindly to userspace solely
> as a boolean hwcap.
Hi Will,

Nasty, so now I'm curious. Any thoughts on how to expose what regions it is appropriate
for?  I can think of various heavy weight options but wondering if there is a simple
solution.

Jonathan
> 
> Will
>