linux-kernel - Re: [PATCH 4/9] x86/alternative: Implement .retpoline

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Date:   Tue, 19 Oct 2021 11:40:56 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alexander Lobakin <alobakin@...me>
Cc:     Borislav Petkov <bp@...en8.de>, x86@...nel.org,
        jpoimboe@...hat.com, andrew.cooper3@...rix.com,
        linux-kernel@...r.kernel.org, alexei.starovoitov@...il.com,
        ndesaulniers@...gle.com
Subject: Re: [PATCH 4/9] x86/alternative: Implement .retpoline_sites support

On Mon, Oct 18, 2021 at 11:06:35PM +0000, Alexander Lobakin wrote:
> From: Peter Zijlstra <peterz@...radead.org>
> Date: Fri, 15 Oct 2021 18:56:35 +0200
> 
> Hi,
> 
> Gave it a spin with Clang/LLVM, and

Just your normal clang build, not some fancy LTO ? eg.  make CC=clang.


> > > > +	target = addr + insn->length + insn->immediate.value;
> > > > +	reg = (target - &__x86_indirect_thunk_rax) /
> > > > +	      (&__x86_indirect_thunk_rcx - &__x86_indirect_thunk_rax);
> 
> this triggers
> 

> > > > +
> > > > +	if (WARN_ON_ONCE(reg & ~0xf))
> > > > +		return -1;
> 
> this:
> 
> WARN in patch_retpoline:408: addr pcibios_scan_specific_bus+0x196/0x200, op 0xe8, reg 0xb88ca
> WARN in patch_retpoline:408: addr xen_pv_teardown_msi_irqs+0x8d/0x120, op 0xe8, reg 0xb88ca
> WARN in patch_retpoline:408: addr __mptcp_sockopt_sync+0x7e/0x200, op 0xe8, reg 0xb88ca
> [...]
> (thousands of them, but op == 0xe8 && reg == 0xb88ca are always the same)
> 
> I know this reg calculation is about to be replaced, but anyway ;)

Well, I was sorta hoping to keep that with something like:

  https://lkml.kernel.org/r/YWgA+vbWCdGLZhq5@hirez.programming.kicks-ass.net

Anyway, let me try and reproduce.