linux-kernel - Re: [PATCH -tip 3/6 V4.1] x86: instruction decorder API

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Message-ID: <49DA8857.8030607@zytor.com>
Date:	Mon, 06 Apr 2009 15:55:19 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Jim Keniston <jkenisto@...ibm.com>
CC:	Masami Hiramatsu <mhiramat@...hat.com>,
	Ingo Molnar <mingo@...e.hu>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Andi Kleen <andi@...stfloor.org>, kvm@...r.kernel.org,
	Steven Rostedt <rostedt@...dmis.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	systemtap-ml <systemtap@...rces.redhat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Vegard Nossum <vegard.nossum@...il.com>,
	Avi Kivity <avi@...hat.com>, Roland McGrath <roland@...hat.com>
Subject: Re: [PATCH -tip 3/6 V4.1] x86: instruction decorder API

Jim Keniston wrote:
> 
> For user-space probing, we've been concentrating on native-built
> executables.  Am I correct in thinking that we'll see 16-bit or V86 mode
> only on legacy apps built elsewhere?  In any case, it only makes sense
> to build on the kvm folks' work in this regard.
> 

That's a fair assumption; you will of course need to test it and take
appropriate action if it doesn't pan out.

> 
> As noted, the INAT tables follow the kvm model of one fat bitmap of
> attributes per opcode, rather than the kprobes/uprobes model of one or
> two 256-bit tables per attribute.  (This latter approach was due to the
> gradual accumulation of tables over the years.)
> 
> I like the bitmap-per-opcode approach because it's relatively easy to
> see in one place everything you're saying about a particular opcode.
> But with all the potential clients for this service, it's not clear that
> we'll get by with a single bitmap for every opcode.  (x86 kvm uses 32
> bits per opcode, I think, and the INAT tables use 10.  Seems like we
> could overrun 64 bits pretty quickly.)  So I guess that means we'll have
> to get a little creative as to how we expose these attribute sets to the
> client.
> 

This is another very good reason to use an instruction table which is
preprocessed into a usable format: it means that if the internal data
structures change -- and they almost certainly will have to at some
point -- the raw data isn't lost.

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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