linux-kernel.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
From: Peter Zijlstra <peterz@infradead.org>
To: Nick Desaulniers <ndesaulniers@google.com>
Cc: Nathan Chancellor <nathan@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	x86@kernel.org, linux-kernel@vger.kernel.org,
	llvm@lists.linux.dev
Subject: Re: [PATCH] x86/build: Move '-mindirect-branch-cs-prefix' out of GCC-only block
Date: Thu, 18 Aug 2022 09:45:39 +0200	[thread overview]
Message-ID: <Yv3uI/MoJVctmBCh@worktop.programming.kicks-ass.net> (raw)
In-Reply-To: <CAKwvOd=0R76r6YqNiOhGNt_XrA_t_ZbuBbNNdNXV4CWpDy0+Bg@mail.gmail.com>

On Wed, Aug 17, 2022 at 03:08:01PM -0700, Nick Desaulniers wrote:
> On Wed, Aug 17, 2022 at 11:56 AM Nathan Chancellor <nathan@kernel.org> wrote:
> >
> > LLVM 16 will have support for this flag so move it out of the GCC-only
> > block to allow LLVM builds to take advantage of it.
> >
> > Link: https://github.com/ClangBuiltLinux/linux/issues/1665
> > Link: https://github.com/llvm/llvm-project/commit/6f867f9102838ebe314c1f3661fdf95700386e5a
> > Signed-off-by: Nathan Chancellor <nathan@kernel.org>
> 
> Thanks for the patch, I also boot tested this (in virtual guests for
> both 64b and 32b).
> 
> Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
> Tested-by: Nick Desaulniers <ndesaulniers@google.com>
> 
> Peter, is there anything else special about these prefixes needed to
> make use of them?

The thing to do is boot with (warning, *very* verbose):

	"spectre_v2=retpoline,lfence debug-alternative"

and observe that the retpoline sites all replace:

	"cs call __x86_indirect_thunk_r11" (6 bytes)

with:

	"lfence; jmp *%r11" (6 bytes)


This being clang, you'll ofcourse still have a few weird:

	"Jcc.d32 __x86_indirect_thunk_r11"

sites that will not be able to be replaced, but there's nothing I can do
about that other than to continue to encourage y'all to stop emitting
them ;-)

  reply	other threads:[~2022-08-18  7:46 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-08-17 18:54 [PATCH] x86/build: Move '-mindirect-branch-cs-prefix' out of GCC-only block Nathan Chancellor
2022-08-17 22:08 ` Nick Desaulniers
2022-08-18  7:45   ` Peter Zijlstra [this message]
2022-08-18  8:00     ` Peter Zijlstra
2022-09-30 18:10     ` Nick Desaulniers
2022-10-04 21:27     ` Nick Desaulniers
2022-10-06 17:56       ` Nick Desaulniers
2022-08-18  7:49 ` Peter Zijlstra

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=Yv3uI/MoJVctmBCh@worktop.programming.kicks-ass.net \
    --to=peterz@infradead.org \
    --cc=bp@alien8.de \
    --cc=dave.hansen@linux.intel.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=llvm@lists.linux.dev \
    --cc=mingo@redhat.com \
    --cc=nathan@kernel.org \
    --cc=ndesaulniers@google.com \
    --cc=tglx@linutronix.de \
    --cc=x86@kernel.org \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body. 
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).