Message-ID: <4A2812E3.6090108@novell.com>
Date: Thu, 04 Jun 2009 14:30:59 -0400
From: Gregory Haskins <ghaskins@novell.com>
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To: Avi Kivity <avi@redhat.com>
CC: "Michael S. Tsirkin" <mst@redhat.com>, linux-kernel@vger.kernel.org,
       agraf@suse.de, pmullaney@novell.com, pmorreale@novell.com,
       anthony@codemonkey.ws, rusty@rustcorp.com.au, netdev@vger.kernel.org,
       kvm@vger.kernel.org, bhutchings@solarflare.com, andi@firstfloor.org,
       gregkh@suse.de, herber@gondor.apana.org.au, chrisw@sous-sol.org,
       shemminger@vyatta.com
Subject: Re: [RFC PATCH v2 03/19] vbus: add connection-client helper infrastructure
References: <20090409155200.32740.19358.stgit@dev.haskins.net> <20090409163057.32740.27829.stgit@dev.haskins.net> <20090604180611.GA16387@redhat.com> <4A280FE2.2090101@novell.com> <4A281161.8010109@redhat.com>
In-Reply-To: <4A281161.8010109@redhat.com>
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Avi Kivity wrote:
> Gregory Haskins wrote:
>
>  =20
>>> BTW, why did you decide to use get_user_pages?
>>> Would switch_mm + copy_to_user work as well
>>> avoiding page walk if all pages are present?
>>>      =20
>>
>> Well, basic c_t_u() won't work because its likely not "current" if you=

>> are updating the ring from some other task, but I think you have alrea=
dy
>> figured that out based on the switch_mm suggestion.  The simple truth =
is
>> I was not familiar with switch_mm at the time I wrote this (nor am I
>> now).  If this is a superior method that allows you to acquire
>> c_t_u(some_other_ctx) like behavior, I see no problem in changing.  I
>> will look into this, and thanks for the suggestion!
>>  =20
>
> copy_to_user() is significantly faster than get_user_pages() + kmap()
> + memcmp() (or their variants).
>

Oh, I don't doubt that (in fact, I was pretty sure that was the case
based on some of the optimizations I could see in studying the c_t_u()
path).  I just didn't realize there were other ways to do it if its a
non "current" task. ;)

I guess the enigma for me right now is what cost does switch_mm have?=20
(Thats not a slam against the suggested approach...I really do not know
and am curious).

As an aside, note that we seem to be reviewing v2, where v3 is really
the last set I pushed.  I think this patch is more or less the same
across both iterations, but FYI that I would recommend looking at v3
instead.

-Greg


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