Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Fri, 29 Oct 2021 14:43:10 +0200 (CEST)
From:   Miroslav Benes <mbenes@suse.cz>
To:     Peter Zijlstra <peterz@infradead.org>
cc:     x86@kernel.org, jpoimboe@redhat.com, andrew.cooper3@citrix.com,
        linux-kernel@vger.kernel.org, alexei.starovoitov@gmail.com,
        ndesaulniers@google.com, bpf@vger.kernel.org
Subject: Re: [PATCH v3 00/16] x86: Rewrite the retpoline rewrite logic
In-Reply-To: <20211026120132.613201817@infradead.org>
Message-ID: <alpine.LSU.2.21.2110291442190.21447@pobox.suse.cz>
References: <20211026120132.613201817@infradead.org>
User-Agent: Alpine 2.21 (LSU 202 2017-01-01)
MIME-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Precedence: bulk

On Tue, 26 Oct 2021, Peter Zijlstra wrote:

> Hi,
> 
> These patches rewrite the way retpolines are rewritten. Currently objtool emits
> alternative entries for most retpoline calls. However trying to extend that led
> to trouble (ELF files are horrid).
> 
> Therefore completely overhaul this and have objtool emit a .retpoline_sites
> section that lists all compiler generated retpoline thunk calls. Then the
> kernel can do with them as it pleases.
> 
> Notably it will:
> 
>  - rewrite them to indirect instructions for !RETPOLINE
>  - rewrite them to lfence; indirect; for RETPOLINE_AMD,
>    where size allows (boo clang!)
> 
> Specifically, the !RETPOLINE case can now also deal with the clang-special
> conditional-indirect-tail-call:
> 
>   Jcc __x86_indirect_thunk_\reg.
> 
> Finally, also update the x86 BPF jit to catch up to recent times and do these
> same things.
> 
> All this should help improve performance by removing an indirection.
> 
> Patches can (soon) be found here:
> 
>   git://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git objtool/core
> 
> Changes since v2:
> 
>  - rewrite the __x86_indirect_thunk_array[] stuff again
>  - rewrite the retpoline,amd rewrite logic, it now also supports
>    rewriting the Jcc case, if the original instruction is long enough, but
>    more importantly, it's simpler code.
>  - bpf label simplification patch
>  - random assorted cleanups
>  - actually managed to get bpf selftests working

It is already in tip, but FWIW the objtool changes look good to me

Reviewed-by: Miroslav Benes <mbenes@suse.cz>

M