On 1/15/19 4:44 AM, David Laight wrote: > Once this is done it might be worth while adding a parameter to > kernel_fpu_begin() to request the registers only when they don't > need saving. > This would benefit code paths where the gains are reasonable but not massive. > > The return value from kernel_fpu_begin() ought to indicate which > registers are available - none, SSE, SSE2, AVX, AVX512 etc. > So code can use an appropriate implementation. > (I've not looked to see if this is already the case!) Yeah, it would be sane to have both a mask passed, and returned, say: got = kernel_fpu_begin(XFEATURE_MASK_AVX512, NO_XSAVE_ALLOWED); if (got == XFEATURE_MASK_AVX512) do_avx_512_goo(); else do_integer_goo(); kernel_fpu_end(got) Then, kernel_fpu_begin() can actually work without even *doing* an XSAVE: /* Do we have to save state for anything in 'ask_mask'? */ if (all_states_are_init(ask_mask)) return ask_mask; Then kernel_fpu_end() just needs to zero out (re-init) the state, which it can do with XRSTORS and a careful combination of XSTATE_BV and the requested feature bitmap (RFBM). This is all just optimization, though.
