linux-kernel - Re: [PATCH v7 2/8] x86/flush_tlb: try flush_tlb_single one by one in flush_tlb

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Message-Id: <4FBE1F2A0200007800085C72@nat28.tlf.novell.com>
Date:	Thu, 24 May 2012 10:44:42 +0100
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Alex Shi" <alex.shi@...el.com>
Cc:	<borislav.petkov@....com>, <arnd@...db.de>,
	<a.p.zijlstra@...llo.nl>, <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>, <luto@....edu>,
	<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>,
	<hpa@...or.com>
Subject: Re: [PATCH v7 2/8] x86/flush_tlb: try flush_tlb_single one by
 one in flush_tlb_range

>>> On 24.05.12 at 10:55, Alex Shi <alex.shi@...el.com> wrote:
> On 05/24/2012 04:12 PM, Jan Beulich wrote:
> 
>>>>> On 24.05.12 at 08:41, Alex Shi <alex.shi@...el.com> wrote:
>>> So, the following change should be more safe for PV?
>>>
>>> -       if (va == TLB_FLUSH_ALL) {
>>> -               args->op.cmd = MMUEXT_TLB_FLUSH_MULTI;
>>> -       } else {
>>> -               args->op.cmd = MMUEXT_INVLPG_MULTI;
>>> -               args->op.arg1.linear_addr = va;
>>> -       }
>>> +       args->op.cmd = MMUEXT_TLB_FLUSH_MULTI;
>> 
>> This would be safe ...
>> 
>>> +       if (start != TLB_FLUSH_ALL)
>>> +               args->op.arg1.linear_addr = start;
>> 
>> ... and then this superfluous, but it'd result in an unconditional
>> full TLB flush. When start and end (or perhaps end-1, assuming
>> end is not inclusive) are on the same page, MMUEXT_INVLPG_MULTI
>> should be used; MMUEXT_TLB_FLUSH_MULTI might need to be
>> used in all other cases, unless you want to split multi-page, non-
>> global invalidations into multiple MMUEXT_INVLPG_MULTI-s (which
>> would appear to be what the whole patch aims at).
> 
> 
>         args->op.cmd = MMUEXT_TLB_FLUSH_MULTI;
> -       if (start != TLB_FLUSH_ALL)
> +       if (start != TLB_FLUSH_ALL && (end - start) < PAGE_SIZE) {

Assuming 'end' is not inclusive (you didn't clarify that),

       if (start != TLB_FLUSH_ALL && (end - start) <= PAGE_SIZE) {

(and only if 'start' is guaranteed to be page aligned).

> +               args->op.cmd = MMUEXT_INVLPG_MULTI;
>                 args->op.arg1.linear_addr = start;
> +       }
> 
> So, above it correct code for xen?
> As to the xen optimisation of flush range, it is may better to be done
> in a separate patch.

Agreed.

Jan

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