Messages in this thread | | | Subject | Re: [RFC] Adding A64FX hardware prefetch sysfs interface | From | James Morse <> | Date | Fri, 11 Jun 2021 19:03:21 +0100 |
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Hello!
(CC: +Catalin and Will)
On 07/06/2021 09:11, Borislav Petkov wrote: > (not trimming the mail so that ARM folks can see the whole thing) > > On Mon, Jun 07, 2021 at 01:39:21AM +0000, tarumizu.kohei@fujitsu.com wrote: >> Hello >> >> I'm Kohei Tarumizu from Fujitsu Limited. >> >> Fujitsu A64FX processor implements a vendor specific function, the HPC extensions[1]. >> A64FX has some registers for HPC extensions. >> We would like to use the register IMP_PF_STREAM_DETECT_CTRL_EL0 for tuning the hardware prefetch, but it's not accessible from userspace. >> We are considering to implement a kernel common interface via sysfs as a way to control IMP_PF_STREAM_DETECT_CTRL_EL0 from userspace.
>> FYI, A64FX also has registers (e.g. IMP_PF_INJECTION_*) to control the behavior of the hardware prefetch from the software using "HPC tag address override", but this time we don't considered. >> >> [1]https://github.com/fujitsu/A64FX/tree/master/doc/ >> A64FX_Specification_HPC_Extension_v1_EN.pdf
While this is initially about sysfs, don't you need the 'HPC tag address override' to be enabled for this to be useful? I don't think that feature can be managed by a driver:
'HPC tag address override' changes the top byte of all user-space pointers from being ignored (as they have been since day-1 on arm64) to having implications for the hardware. If I've read the document correctly this affects the prefetch mode and where in the L1/L2 such accesses will be allocated.
This would impact user-space that is using the top-byte for their own purposes. For example hwasan uses this field as a tag it allocates itself: https://clang.llvm.org/docs/HardwareAssistedAddressSanitizerDesign.html Enabling 'HPC tag address override' for all user-space is going to have weird performance effects.
To make this work, I think you'd need a per-process opt-in, and __switch_to() would need to toggle your IMP_FJ_TAG_ADDRESS_CTRL_EL1.TBOx bits. Because its an implementation-defined feature, but the controls can't be confined to a driver, I don't think enabling 'HPC tag address override' is viable.
Is the sysfs information useful without it?
Thanks,
James
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