On Thu, Apr 23, 2020 at 12:18:03PM +0100, Mark Brown wrote: > On Wed, Apr 22, 2020 at 07:00:28PM +0100, Catalin Marinas wrote: > > On Wed, Mar 25, 2020 at 11:50:38AM +0000, Mark Brown wrote: > > > Since BTI is a mandatory feature of v8.5 there is no BTI arch_extension, > > > you can only enable it by moving the base architecture to v8.5. You'd > > > need to use .arch and that feels likely to find us sharp edges to run > > > into. > > > For MTE, .arch armv8-a+memtag won't work since this is only available > > with armv8.5-a. My preference would be to have the highest arch version > > supported by the kernel in the assembler.h file, i.e. ".arch armv8.5-a" > > followed by .arch_extension in each .S file, as needed. > > I think we decided that .arch_extension was too new to be used for > things like the crypto stuff where we still support older toolchains? .arch_extension would be issued conditionally only for features like CONFIG_ARM64_MTE which already have a dependency on a newer toolchain. However, '.arch_extension memtag' is not sufficient for MTE, it needs a prior '.arch armv8.5-a'. > > Forcing .S files to armv8.5 would not cause any problems with > > the base armv8.0 that the kernel image support since it shouldn't change > > the opcodes gas generates. The .S files would use alternatives anyway > > (or simply have code not called). > > We do loose the checking that the assembler does that nobody used a > newer feature by mistake but yeah, shouldn't affect the output. We may need some push/pop_arch macros to contain the supported features. The gas documentation says that .arch_extension may be used multiple times to add or remove extensions. However, I couldn't find a way to remove memtag after adding it (tried -memtag, !memtag, empty string). So I may go with a '.arch armv8.0-a' as a base, followed by temporary setting of '.arch armv8.5-a+memtag' (and hope we don't need combinations of such extensions). > > The inline asm is slightly more problematic, especially with the clang > > builtin assembler which goes in a single pass. But we could do something > > similar to what we did with the LSE atomics and raising the base of the > > inline asm to armv8.5 (or 8.6 etc., whatever we need in the future). > > FWIW I did something different to this for BTI so I wasn't using the > instructions directly so I was going to abandon this series. I can't work around this easily for MTE, there are more instructions with register encoding. I'll see if the push/pop idea works or just leave it to whoever does the next feature, figure out how it interacts with MTE ;). -- Catalin
