On Mon, Feb 21, 2022 at 11:10:34PM +0000, Mark Brown wrote: > On Mon, Feb 21, 2022 at 07:24:59PM +0000, Catalin Marinas wrote: > > On Mon, Feb 21, 2022 at 03:01:03PM +0000, Mark Brown wrote: > > > We do run the kernel in streaming mode - entering the kernel through a > > > syscall or preemption will not change the streaming mode state, and we > > > need to be in streaming mode in order to save or restore the register > > > state for streaming mode. In particular we need FA64 enabled for EL1 in > > > order to context switch FFR when in streaming mode, without it we'll > > > generate an exception when we execute the rdffr or wrffr. We don't do > > > any real floating point work in streaming mode but we absolutely need to > > > run in streaming mode and only exit streaming mode when restoring a > > > context where it is disabled, when using floating point in the kernel or > > > when idling the CPU. > > > So, IIUC, for Linux it is mandatory that FEAT_SME_FA64 is supported, > > otherwise we won't be able to enable SME. Does the architecture say > > The feature is not mandatory and we do not require it for Linux. It is > expected that many implementations will choose to not support FA64. > > The only impact it has on the kernel is that if it's present then we > need to enable it for each EL and then context switch FFR in streaming > mode, the code is there to do that conditionally already. OK, I get it. So FFR is only present if FA64 is supported. > This is actually a bit awkward for not disabling streaming mode when we > do a syscall since the disabled instructions include the FPSMID mov > vector, vector instruction which we currently use to zero the high bits > of the Z registers. That issue goes away if the optimisations I've got > for relaxed flushing of the non-shared SVE state that we discussed in > relation to syscall-abi get merged, though it'd still be there if we add > a sysctl to force flushing. This is a solvable problem though, even if > we have to use a less efficient sequence to flush in streaming mode. I guess the simplest is to just disable streaming mode on syscall. The C library would mark the syscall wrappers as not streaming compatible, so whoever is calling them might disable SM anyway. So I think your original proposal in the ABI doc is fine (I just need the libc people to confirm ;)). -- Catalin
