On Wed, Jun 26, 2019 at 03:14:05PM -0700, Nick Desaulniers wrote: > On Wed, Jun 26, 2019 at 1:49 AM Peter Zijlstra <pet...@infradead.org> wrote: > > > > On Tue, Jun 25, 2019 at 11:15:57AM -0700, Nick Desaulniers wrote: > > > > > Unreleased versions of Clang built from source can; > > > > I've bad experiences with using unreleased compilers; life is too short. > > Yes; but before release is when they need the help the most in order > for testing to find regressions. > > > > > > We're currently planning multiple output constraint support w/ asm > > > goto, and have recently implemented things like > > > __GCC_ASM_FLAG_OUTPUTS__. > > > > That's good to hear. > > > > > If there's other features that we should > > > start implementing, please let us know. > > > > If you've got any ideas on how to make this: > > > > > > https://lkml.kernel.org/r/20190621120923.gt3...@hirez.programming.kicks-ass.net > > > > work, that'd be nice. Basically I wanted the asm goto to emit a 2 or 5 > > byte JMP/NOP depending on the displacement size. We can trivially get > > JMP right by using: > > > > jmp \l_yes > > > > and letting the assembler sort it, but getting the NOP right has so far > > eluded me: > > > > .if \l_yes - (. + 2) < 127 > > .byte 0x66, 0x90 > > .else > > .byte STATIC_KEY_INIT_NOP > > .endif > > > > doesn't work. We can ofcourse unconditionally emit the JMP and then > > rewrite the binary afterward, and replace the emitted jumps with the > > right size NOP, but that's a bit yuck. > > > > Once it emits the variable size instruction consistently, we can update > > the patching side to use the same condition to select the new > > instruction (and fix objtool). > > Not sure; the assembler directives and their requirements aren't > something I'm too familiar with.
Josh came up with the following: + /* If the jump target is close, do a 2-byte nop: */ + ".skip -(%l[l_yes] - 1b <= 126), 0x66\n" + ".skip -(%l[l_yes] - 1b <= 126), 0x90\n" + /* Otherwise do a 5-byte nop: */ + ".skip -(%l[l_yes] - 1b > 126), 0x0f\n" + ".skip -(%l[l_yes] - 1b > 126), 0x1f\n" + ".skip -(%l[l_yes] - 1b > 126), 0x44\n" + ".skip -(%l[l_yes] - 1b > 126), 0x00\n" + ".skip -(%l[l_yes] - 1b > 126), 0x00\n" Which is a wonderfully gruesome hack :-) So I'll be playing with that for a bit.
