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Message-ID: <20150822132131.GA3982@gmail.com>
Date: Sat, 22 Aug 2015 15:21:31 +0200
From: Ingo Molnar <mingo@...nel.org>
To: Dave Hansen <dave@...1.net>
Cc: dave.hansen@...ux.intel.com, linux-kernel@...r.kernel.org,
bp@...en8.de, fenghua.yu@...el.com, hpa@...or.com, x86@...nel.org,
Thomas Gleixner <tglx@...utronix.de>,
Peter Zijlstra <a.p.zijlstra@...llo.nl>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Andy Lutomirski <luto@...nel.org>,
Denys Vlasenko <dvlasenk@...hat.com>
Subject: Re: [PATCH] x86, fpu: correct XSAVE xstate size calculation
* Dave Hansen <dave@...1.net> wrote:
> On 08/08/2015 02:06 AM, Ingo Molnar wrote:
> >>> What sense does it make to have a blob we don't know the exact layout of? How will
> >>> > > debuggers or user-space in general be able to print (and change) the register
> >>> > > values if they don't know the layout?
> >> >
> >> > Ingo, we know the layout. We know where every component is. We know
> >> > how big each component is. This patch does not change the fact that we
> >> > calculate and store that.
> > The patch you submitted blindly trusts the CPU, and I'm uneasy about that for
> > multiple reasons. We can and should do better than that, while still flexibly
> > making use of all CPU capabilities that are offered.
>
> Yes, it blindly trusts the CPU. This is precisely* what the *existing*
> code has done since commit dc1e35c6e95 got merged in 2008. Do you have
> some specific concern with the compact format that makes you want to
> stop blindly trusting the CPU after 7 years?
Yes, the fact that 'compact format' probably never worked well and we had to
revert use of it.
> I know what you want now (I've coded up half of it already), but I've not got
> the foggiest idea why other than pure paranoia.
There were multiple bugs in this code so some amount of paranoia is justified.
Thanks,
Ingo
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