linux-kernel - Re: 2.6.26, PAT and AMD family 6

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Wed, 07 May 2008 17:31:32 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Rene Herman <rene.herman@...access.nl>,
	Thomas Gleixner <tglx@...utronix.de>,
	Adrian Bunk <bunk@...nel.org>,
	Yinghai Lu <yhlu.kernel@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	akpm@...ux-foundation.org, Pavel Machek <pavel@...e.cz>
Subject: Re: 2.6.26, PAT and AMD family 6

Linus Torvalds wrote:
> 
> On Wed, 7 May 2008, H. Peter Anvin wrote:
>>> What a lovely way of syncing reality with your definitions. The kernel
>>> _does_ see that my CPU features PAT, it just refuses to use it because
>>> it doesn't trust it enough. Vital difference. Maybe not to the kernel,
>>> but definitely to me, the user. /proc/cpuinfo is a user interface.
>>>
>> No, /proc/cpuinfo is informing the user about the kernel's view of the CPU.
>> It has always been the "cooked" view of CPUID, whether or not you like it.
> 
> Indeed. If people want to know what the CPU itself reports, they should 
> just use the "cpuid" instruction directly. The /proc/cpuinfo file has 
> been a window into the kernels notion of what is going on, and disabling 
> kernel features have disabled the bits in that file as appropriate (eg 
> "clearcpuid=xyzzy" and "mem=nopentium" etc).
> 
> Of course, the *common* case is that the two will match 100% in features. 
> But the kernel view has some of its very own set of CPU features too.
> 
> And it is possible that the "PAT as the kernel sees it" should be a 
> *separate* bit from "PAT as the cpu itself reports to support it". We do 
> that for quite a lot of the "synthesized" features, eg there is the "TSC" 
> feature as the CPU reports it, and then we have the "CONSTANT_TSC" feature 
> that is the kernel version of it that says that we have a TSC _and_ it 
> runs at a constant rate too (or the "UP" bit, or the "Mfence synchronizes 
> RDTSC" bit etc etc).
> 
> So it's possible that we should do the same thing for PAT - and allow 
> people to see the CPU view _and_ the kernel view as two separate issues. 
> In many ways that would be the logical thing to do. Hmm?
> 

Well, there *is* support for that - all the raw information is there in 
/dev/cpu/*/cpuid.  There are other reasons why /proc/cpuinfo is the 
wrong interface to try to get the "real" CPUID information - we only 
report CPU features that the kernel knows about; bits we don't, if we 
can decode them at all, we just don't show.

	-hpa
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/