linux-kernel - Re: Do the x86 kernel entry points need an xabort on TSX cpus?

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Date:	Fri, 10 Feb 2012 18:51:13 +0000
From:	Simon Farnsworth <simon@...nz.org.uk>
To:	linux-kernel@...r.kernel.org
Subject: Re: Do the x86 kernel entry points need an xabort on TSX cpus?

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Andy Lutomirski wrote:

> On Fri, Feb 10, 2012 at 9:18 AM, H. Peter Anvin <hpa@...ux.intel.com>
> wrote:
>> On 02/09/2012 11:40 PM, Andy Lutomirski wrote:
>>>
>>> [...]
>>>
>>> - Ring transitions: SYSENTER, SYSCALL, SYSEXIT, and SYSRET.
>>>
>>> I suspect that many bits of the kernel expect that things they do
>>> won't unhappen.  For example, it could be fun to do:
>>>
>>
>> That's why entering the kernel will cause an abort.  In other words, you
>> will ALWAYS abort when you do a read(), and you will never reach your
>> _xabort().
> 
> Is that architecturally guaranteed?  (My manual suggests that it's
> specifically *not* guaranteed, which is surprising.)
> 
> --Andy
My understanding of the architecture manual's wording (which is a bit 
clumsy) is that they want to leave themselves wiggle room just in case they 
work out a way to do any of these things without requiring an abort.

If, for example, Intel add an MSR for SYSENTER that's used to go to a 
different entrypoint if you're mid-transaction, you've suddenly broken all 
code that assumes that SYSENTER triggers an abort - instead, some SYSENTERs 
trigger an abort (as the kernel does the XABORT), while others don't.

Current implementations appear to always abort on ring transition, though.
- -- 
Simon Farnsworth
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