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Date:   Fri, 22 Jul 2022 13:03:19 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     David Laight <David.Laight@aculab.com>
Cc:     'Linus Torvalds' <torvalds@linux-foundation.org>,
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        Alyssa Milburn <alyssa.milburn@linux.intel.com>,
        Jann Horn <jannh@google.com>, "H.J. Lu" <hjl.tools@gmail.com>,
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        Daniel Borkmann <daniel@iogearbox.net>,
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        Kees Cook <keescook@chromium.org>
Subject: Re: [patch 00/38] x86/retbleed: Call depth tracking mitigation
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On Thu, Jul 21, 2022 at 10:01:12PM +0000, David Laight wrote:

> Since: "If the callee is a variadic function, then the number of floating
> point arguments passed to the function in vector registers must be provided
> by the caller in the AL register."
> 
> And that that never happens in the kernel you can use %eax instead
> of %r10d.

Except there's the AMD BTC thing and we should (compiler patch seems
MIA) have an unconditional: 'xor %eax,%eax' in front of every function
call.

(The official mitigation strategy was CALL; LFENCE IIRC, but that's so
horrible nobody is actually considering that)

Yes, the suggested sequence ends with rax being zero, but since we start
the speculation before that result is computed that's not good enough I
suspect.