linux-kernel - Re: [PATCH v3 3/4] x86/alternative: Rewrite optimize

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Date:   Wed, 8 Feb 2023 21:44:04 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andrew.Cooper3@...rix.com
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, mhiramat@...nel.org,
        kirill.shutemov@...ux.intel.com, jpoimboe@...hat.com
Subject: Re: [PATCH v3 3/4] x86/alternative: Rewrite optimize_nops() some

On Wed, Feb 08, 2023 at 09:36:24PM +0100, Peter Zijlstra wrote:
> On Wed, Feb 08, 2023 at 09:29:23PM +0100, Peter Zijlstra wrote:
> 
> > As is, there were only a hand full of NOPs that turned into this jmp.d8
> > thing on the defconfig+kvm_guest.config I build to test this -- it is by
> > no means a common thing. And about half of them would be gone by
> > extending the max nop length to at least 10 or so.
> > 
> > In fact, I did that patch once, lemme see if I still have it...
> 
> Even still applies too, lemme go test that.

Works as advertised. Next up are 8 12 byte nopes and 8 13 byte nops,
after that we're left with:

[   11.584069] SMP alternatives: ffffffff82000095: [0:20) optimized NOPs: eb 12 cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc
[   11.588621] SMP alternatives: ffffffff810026dc: [0:44) optimized NOPs: eb 2a cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cccc cc cc cc cc cc cc cc cc
[   11.590068] SMP alternatives: ffffffff820001f3: [0:20) optimized NOPs: eb 12 cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc
[   11.720069] SMP alternatives: ffffffff8200189f: [0:20) optimized NOPs: eb 12 cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc
[   11.731069] SMP alternatives: ffffffff820019ae: [0:20) optimized NOPs: eb 12 cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc
[   11.738069] SMP alternatives: ffffffff82001a4a: [0:20) optimized NOPs: eb 12 cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc
[   11.746069] SMP alternatives: ffffffff82001b2d: [0:20) optimized NOPs: eb 12 cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc
[   11.766069] SMP alternatives: ffffffff82001d14: [0:20) optimized NOPs: eb 12 cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc
[   11.770069] SMP alternatives: ffffffff82001dd5: [0:20) optimized NOPs: eb 12 cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc
[   11.779069] SMP alternatives: ffffffff82001f35: [0:20) optimized NOPs: eb 12 cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc

Which is all pretty big...

GNU as seems to go to .nops 12. 13 gives me a single byte nop and a 12.
That's also exactly the 3 prefix limit, after which some uarchs start
taking heavy decode penalties.

Let me go extend that patch of mine to cover 12. Also, instead of
printing hex addresses, lemme go print symbol+off things.