linux-kernel - Re: [patch 13/17] Immediate Values

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Date:	Wed, 9 Apr 2008 19:15:34 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	akpm@...ux-foundation.org, Ingo Molnar <mingo@...e.hu>,
	linux-kernel@...r.kernel.org, Andi Kleen <andi@...stfloor.org>,
	Rusty Russell <rusty@...tcorp.com.au>, Andi Kleen <ak@....de>,
	Chuck Ebbert <cebbert@...hat.com>,
	Christoph Hellwig <hch@...radead.org>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Adrian Bunk <bunk@...sta.de>,
	Alexey Dobriyan <adobriyan@...il.com>, akpm@...l.org
Subject: Re: [patch 13/17] Immediate Values - x86 Optimization (updated)

* H. Peter Anvin (hpa@...or.com) wrote:
> Mathieu Desnoyers wrote:
>> Hrm, since even the nmi-safe version supports REX-prefixed instructions,
>> there is no need for an =q constraint on single-byte immediate values
>> anymore. (thanks to your "discard" section used in the nmi-safe version)
>
> WRONG!
>
> Using =r for single-byte values is incorrect for 32-bit code -- that would 
> permit %spl, %bpl, %sil, %dil which are illegal in 32-bit mode. That is not 
> the incorrect bit, it's the description that is confused.
>
> 	-hpa

Ah, right. I remembered there was something that made us use =q, but
could not remember what. I'll describe it correctly.

Therefore, this updated patch is bogus. The original one was ok, given
that we change the header to :


Immediate Values - x86 Optimization

x86 optimization of the immediate values which uses a movl with code patching
to set/unset the value used to populate the register used as variable source.

Note : a movb needs to get its value froma =q constraint.

Quoting "H. Peter Anvin" <hpa@...or.com>

Using =r for single-byte values is incorrect for 32-bit code -- that would  
permit %spl, %bpl, %sil, %dil which are illegal in 32-bit mode. 

Changelog:
- Use text_poke_early with cr0 WP save/restore to patch the bypass. We are doing
  non atomic writes to a code region only touched by us (nobody can execute it
  since we are protected by the imv_mutex).
- Put imv_set and _imv_set in the architecture independent header.
- Use $0 instead of %2 with (0) operand.
- Add x86_64 support, ready for i386+x86_64 -> x86 merge.
- Use asm-x86/asm.h.
- Bugfix : 8 bytes 64 bits immediate value was declared as "4 bytes" in the
  immediate structure.
- Change the immediate.c update code to support variable length opcodes.
- Vastly simplified, using a busy looping IPI with interrupts disabled.
  Does not protect against NMI nor MCE.
- Pack the __imv section. Use smallest types required for size (char).
- Use imv_* instead of immediate_*.


Mathieu

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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