linux-kernel - Re: [PATCH] Add a text

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Message-ID: <CA+55aFzTppZ+uhZ+mfHYmmJ9+aboEMUQ9PUfW63BTA-z+1nWqQ@mail.gmail.com>
Date:	Wed, 27 Nov 2013 15:04:41 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Borislav Petkov <bp@...en8.de>,
	Andy Lutomirski <luto@...capital.net>,
	Andi Kleen <andi@...stfloor.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Andi Kleen <ak@...ux.intel.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] Add a text_poke syscall v2

On Wed, Nov 27, 2013 at 2:31 PM, H. Peter Anvin <hpa@...or.com> wrote:
> No, we're not... sysexit/sysret doesn't count.

So sysexit/sysret doesn't count as a serializing instruction, no. But
it doesn't need to, because *self*-modifying code doesn't need a
serializing instruction, only a branch. It's only *cross*-modifying
code that needs a serializing instruction.

So the IPI is sufficient for the cross-modifying case, and the sysret
is sufficient for the self-modifying case. And we also don't need to
worry about "what happens if we schedule to another CPU, and
self-modifying becomes cross-modifying", because the scheduling will
then do the serializing instruction.

So IPI for other CPU's (limited to the mm-mask) and just a system call
for local CPU should be perfectly fine.

          Linus
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