On October 23, 2020 2:52:16 PM PDT, David Laight <david.lai...@aculab.com> wrote: >From: Linus Torvalds >> Sent: 23 October 2020 22:11 >> >> On Fri, Oct 23, 2020 at 2:00 PM <h...@zytor.com> wrote: >> > >> > There is no same reason to mess around with hacks when we are >talking about dx:ax, though. >> >> Sure there is. >> >> "A" doesn't actually mean %edx:%eax like you seem to think. >> >> It actually means %eax OR %edx, and then if given a 64-bit value, it >> will use the combination (with %edx being the high bits). >> >> So using "A" unconditionally doesn't work - it gives random behavior >> for 32-bit (or smaller) types. >> >> Or you'd have to cast the value to always be 64-bit, and have the >> extra code generation. >> >> IOW, an unconditional "A" is wrong. >> >> And the alternative is to just duplicate things, and go back to the >> explicit size testing, but honestly, I really think that's much worse >> than relying on a documented feature of "register asm()" that gcc >> _documents_ is for this kind of inline asm use. > >Could do_put_user() do an initial check for 64 bit >then expand a different #define that contains the actual >code passing either "a" or "A" for the constriant. > >Apart from another level of indirection nothing is duplicated. > > David > >- >Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, >MK1 1PT, UK >Registration No: 1397386 (Wales)
Maybe #define ASM_AX64 or some such. -- Sent from my Android device with K-9 Mail. Please excuse my brevity.
