Re: [GIT PULL] objtool/urgent for v5.15-rc4

[Paolo Bonzini]

On 04/10/21 01:02, Josh Poimboeuf wrote:
> That said, I have no idea what's going in that code or why
> kvm_fastop_exception() is clearing %esi.

It's handled here (which definitely qualifies as "funky sh*t"):

        asm("push %[flags]; popf; " CALL_NOSPEC " ; pushf; pop %[flags]\n"
            : "+a"(ctxt->dst.val), "+d"(ctxt->src.val), [flags]"+D"(flags),
              [thunk_target]"+S"(fop), ASM_CALL_CONSTRAINT
            : "c"(ctxt->src2.val));

        ctxt->eflags = (ctxt->eflags & ~EFLAGS_MASK) | (flags & EFLAGS_MASK);
        if (!fop) /* exception is returned in fop variable */
                return emulate_de(ctxt);

and documented here:

/*
 * fastop functions have a special calling convention:
 *
 * dst:    rax        (in/out)
 * src:    rdx        (in/out)
 * src2:   rcx        (in)
 * flags:  rflags     (in/out)
 * ex:     rsi        (in:fastop pointer, out:zero if exception)
 *
 * Moreover, they are all exactly FASTOP_SIZE bytes long, so functions for
 * different operand sizes can be reached by calculation, rather than a jump
 * table (which would be bigger than the code).
 */

The fastop stuff saves quite a few clock cycles and lines of code, by
avoiding complicated emulation of x86 flags.  I'll check out the .global
annotations, since they are indeed unnecessary.

Paolo