Re: [PATCH] x86, fpu: correct XSAVE xstate size calculation

From: Ingo Molnar <mingo@kernel.org>
To: Dave Hansen <dave@sr71.net>
Cc: dave.hansen@linux.intel.com, linux-kernel@vger.kernel.org,
	bp@alien8.de, fenghua.yu@intel.com, hpa@zytor.com,
	x86@kernel.org, Thomas Gleixner <tglx@linutronix.de>,
	Peter Zijlstra <a.p.zijlstra@chello.nl>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Andy Lutomirski <luto@kernel.org>,
	Denys Vlasenko <dvlasenk@redhat.com>
Subject: Re: [PATCH] x86, fpu: correct XSAVE xstate size calculation
Date: Sat, 22 Aug 2015 15:21:31 +0200	[thread overview]
Message-ID: <20150822132131.GA3982@gmail.com> (raw)
In-Reply-To: <55C91453.5060509@sr71.net>

* Dave Hansen <dave@sr71.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