linux-kernel - Re: [PATCH v7 8/8] x86/tlb: just do tlb flush on one of siblings of SMT

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Date:	Fri, 25 May 2012 08:24:40 +0800
From:	Alex Shi <alex.shi@...el.com>
To:	"H. Peter Anvin" <hpa@...or.com>
CC:	Arjan van de Ven <arjan@...ux.intel.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Lutomirski <luto@....edu>,
	Jan Beulich <JBeulich@...e.com>, borislav.petkov@....com,
	arnd@...db.de, akinobu.mita@...il.com, eric.dumazet@...il.com,
	fweisbec@...il.com, rostedt@...dmis.org, hughd@...gle.com,
	jeremy@...p.org, len.brown@...el.com, tony.luck@...el.com,
	yongjie.ren@...el.com, kamezawa.hiroyu@...fujitsu.com,
	seto.hidetoshi@...fujitsu.com, penberg@...nel.org,
	yinghai@...nel.org, tglx@...utronix.de, akpm@...ux-foundation.org,
	ak@...ux.intel.com, avi@...hat.com, dhowells@...hat.com,
	mingo@...hat.com, riel@...hat.com, cpw@....com, steiner@....com,
	linux-kernel@...r.kernel.org, viro@...iv.linux.org.uk,
	"asit.k.mallick@...el.com" <asit.k.mallick@...el.com>
Subject: Re: [PATCH v7 8/8] x86/tlb: just do tlb flush on one of siblings
 of SMT

On 05/24/2012 11:03 PM, H. Peter Anvin wrote:

> On 05/24/2012 07:32 AM, Alex Shi wrote:
>>>
>>> the TLB pool is shared as physical resource (dynamic or static, that
>>> depends), but each tlb entry will be tagged for which of the two HT
>>> pairs it's for, and on a logical level, they are completely separate as
>>> a result (as they should be)
>>
>> But, why just flush part of SMT doesn't crash kernel on many benchmarks
>> testing? Does it means flush tlb without PCID (doesn't enable in current
>> kernel) will flush both of 'TLB pool'?
>>
>> Oh, lots of questions of the TLB pool details. :) Could you like share
>> the URL of related documents?
>>
> 
> Hang on here... there is a huge difference between what a particular CPU
> implementation does and what is architecturally guaranteed.
> 
> Both wearing my Linux x86 maintainer hat, and wearing my Intel employee
> hat, I want to categorically state that Linux cannot rely on behavior
> that isn't architecturally guaranteed.  Unless we can get an
> architectural guarantee that this elision is safe, it cannot go in.  It
> doesn't work the other way -- the burden of proof is to prove that the
> change is safe, not that the change cannot be proven unsafe.


Understand and thanks for all of your time!

> 
> 	-hpa
> 


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