On October 23, 2020 1:42:39 PM PDT, Andy Lutomirski <l...@kernel.org> wrote: >On Fri, Oct 23, 2020 at 1:32 PM Rasmus Villemoes ><li...@rasmusvillemoes.dk> wrote: >> >> Quoting >> https://gcc.gnu.org/onlinedocs/gcc/Local-Register-Variables.html: >> >> You can define a local register variable and associate it with a >> specified register... >> >> The only supported use for this feature is to specify registers for >> input and output operands when calling Extended asm (see Extended >> Asm). This may be necessary if the constraints for a particular >> machine don't provide sufficient control to select the desired >> register. >> >> On 32-bit x86, this is used to ensure that gcc will put an 8-byte >> value into the %edx:%eax pair, while all other cases will just use >the >> single register %eax (%rax on x86-64). While the _ASM_AX actually >just >> expands to "%eax", note this comment next to get_user() which does >> something very similar: >> >> * The use of _ASM_DX as the register specifier is a bit of a >> * simplification, as gcc only cares about it as the starting point >> * and not size: for a 64-bit value it will use %ecx:%edx on 32 bits >> * (%ecx being the next register in gcc's x86 register sequence), and >> * %rdx on 64 bits. >> >> However, getting this to work requires that there is no code between >> the assignment to the local register variable and its use as an input >> to the asm() which can possibly clobber any of the registers involved >> - including evaluation of the expressions making up other inputs. > >This looks like the patch is an improvement, but this is still IMO >likely to be very fragile. Can we just do the size-dependent "a" vs >"A" selection method instead? Sure, it's a little more code, but it >will be Obviously Correct. As it stands, I can easily see our code >failing on some gcc or clang version and the compiler authors telling >us that we're relying on unsupportable behavior.
Yeah, the reason get_user hacks it is because there is no equivalent to "A" for other register pairs, but not using it for dx:ax is just silly. -- Sent from my Android device with K-9 Mail. Please excuse my brevity.