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

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Date:   Mon, 4 Oct 2021 09:28:08 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Borislav Petkov <bp@...e.de>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Nathan Chancellor <nathan@...nel.org>, x86-ml <x86@...nel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        KVM list <kvm@...r.kernel.org>
Subject: Re: [GIT PULL] objtool/urgent for v5.15-rc4

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