linux-kernel - Re: [PATCH 1/5] x86/entry/32: Clean up enable_sep

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Date:	Thu, 23 Jul 2015 11:42:29 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Brian Gerst <brgerst@...il.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Willy Tarreau <w@....eu>, Borislav Petkov <bp@...en8.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 1/5] x86/entry/32: Clean up enable_sep_cpu to prepare for
 64-bit merge

On Thu, Jul 23, 2015 at 11:33 AM, Andy Lutomirski <luto@...capital.net> wrote:
> On Thu, Jul 23, 2015 at 11:01 AM, Andy Lutomirski <luto@...capital.net> wrote:
>> On Thu, Jul 23, 2015 at 10:27 AM, Brian Gerst <brgerst@...il.com> wrote:
>>> On Thu, Jul 23, 2015 at 12:02 PM, Andy Lutomirski <luto@...capital.net> wrote:
>>>> On Thu, Jul 23, 2015 at 8:56 AM, Steven Rostedt <rostedt@...dmis.org> wrote:
>>>>> On Thu, 23 Jul 2015 08:31:39 -0700
>>>>> Andy Lutomirski <luto@...nel.org> wrote:
>>>>>
>>>>>> Switch from wrmsr to wrmsrl_safe to prepare to merge the 32-bit and
>>>>>> 64-bit code, and use __KERNEL_CS explicitly to initialize
>>>>>> MSR_IA32_SYSENTER_CS.  While we're at it, tweak the whitespace a
>>>>>> bit.
>>>>>
>>>>> Saying "prepare to merge the 32-bit and 64-bit" isn't that informative
>>>>> to why this patch is needed. Can you please describe in the change log
>>>>> what can fault with the wrmsr call, when we do the merge?
>>>>
>>>> Oh...
>>>>
>>>> Nothing can fault in the wrmsr as far as I know, but maybe there's a
>>>> CPU out there that advertises SEP but doesn't have the MSRs.
>>>>
>>>> Ingo, Thomas, want a v2 that explains that the wrmsr_safe is there for
>>>> consistency but that I don't know why 32bit does it?  Another option
>>>> would be to remove the _safe from the 32-bit version.
>>>
>>> There is an erratum with SEP being advertised but not available on
>>> early P6 models, but we already check for that in the cpu setup code.
>>> There shouldn't be a problem with a fault.
>>
>> I can make a v2 that gets rid of the _safe.
>>
>
> Ugh.  wrmsrl_safe is a function but wrmsrl isn't.  I'll fix that and send a v2.
>
> Gah, macros.

On third thought, no v2.  It's *64-bit* CPUs that might want the
_safe.  We aren't currently checking SEP.  Maybe there's a non-SEP AMD
CPU and we never noticed because SEP was never useful on AMD 64-bit
CPUs.

I'll send a followup.

>
> --Andy



-- 
Andy Lutomirski
AMA Capital Management, LLC
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