On Thu, 11 Sep 2025 16:50:14 +0100 Will Deacon <will@xxxxxxxxxx> 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 >
