From: 'Sebastian Andrzej Siewior' > Sent: 15 January 2019 13:15 > On 2019-01-15 12:44:53 [+0000], 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. > > So if saving + FPU code is a small win why not do this always? I was thinking of the case when the cost of the fpu save is greater than the saving. This might be true for (say) a crc on a short buffer. > > 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!) > > Either everything is saved or nothing. So if SSE registers are saved > then AVX512 are, too. (I know that - I've written fpu save code for AVX). I was thinking that the return value would depend on what the cpu supports. In fact, given some talk about big-little cpus it might be worth being able to ask for a specific register set. Potentially that could cause a processor switch. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
