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Date:   Thu, 29 Nov 2018 16:17:45 -0600
From:   Josh Poimboeuf <jpoimboe@redhat.com>
To:     Andy Lutomirski <luto@kernel.org>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Peter Zijlstra <peterz@infradead.org>, X86 ML <x86@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Jason Baron <jbaron@akamai.com>, Jiri Kosina <jkosina@suse.cz>,
        David Laight <David.Laight@aculab.com>,
        Borislav Petkov <bp@alien8.de>, julia@ni.com, jeyu@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [PATCH v2 4/4] x86/static_call: Add inline static call
 implementation for x86-64
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On Thu, Nov 29, 2018 at 02:24:52PM -0600, Josh Poimboeuf wrote:
> On Thu, Nov 29, 2018 at 11:27:00AM -0800, Andy Lutomirski wrote:
> > On Thu, Nov 29, 2018 at 11:08 AM Linus Torvalds
> > <torvalds@linux-foundation.org> wrote:
> > >
> > > On Thu, Nov 29, 2018 at 10:58 AM Linus Torvalds
> > > <torvalds@linux-foundation.org> wrote:
> > > >
> > > > In contrast, if the call was wrapped in an inline asm, we'd *know* the
> > > > compiler couldn't turn a "call wrapper(%rip)" into anything else.
> > >
> > > Actually, I think I have a better model - if the caller is done with inline asm.
> > >
> > > What you can do then is basically add a single-byte prefix to the
> > > "call" instruction that does nothing (say, cs override), and then
> > > replace *that* with a 'int3' instruction.
> > >
> > > Boom. Done.
> > >
> > > Now, the "int3" handler can just update the instruction in-place, but
> > > leave the "int3" in place, and then return to the next instruction
> > > byte (which is just the normal branch instruction without the prefix
> > > byte).
> > >
> > > The cross-CPU case continues to work, because the 'int3' remains in
> > > place until after the IPI.
> > 
> > Hmm, cute.  But then the calls are in inline asm, which results in
> > giant turds like we have for the pvop vcalls.  And, if they start
> > being used more generally, we potentially have ABI issues where the
> > calling convention isn't quite what the asm expects, and we explode.
> > 
> > I propose a different solution:
> > 
> > As in this patch set, we have a direct and an indirect version.  The
> > indirect version remains exactly the same as in this patch set.  The
> > direct version just only does the patching when all seems well: the
> > call instruction needs to be 0xe8, and we only do it when the thing
> > doesn't cross a cache line.  Does that work?  In the rare case where
> > the compiler generates something other than 0xe8 or crosses a cache
> > line, then the thing just remains as a call to the out of line jmp
> > trampoline.  Does that seem reasonable?  It's a very minor change to
> > the patch set.
> 
> Maybe that would be ok.  If my math is right, we would use the
> out-of-line version almost 5% of the time due to cache misalignment of
> the address.

BTW, this means that if any of a trampoline's callers crosses cache
boundaries then we won't be able to poison the trampoline.  Which is
kind of sad.

-- 
Josh