linux-kernel - Re: [PATCH 0/7] Support for GB pages in KVM

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Date:	Sun, 29 Mar 2009 14:47:35 +0200
From:	Joerg Roedel <joro@...tes.org>
To:	Avi Kivity <avi@...hat.com>
Cc:	Marcelo Tosatti <mtosatti@...hat.com>,
	Joerg Roedel <joerg.roedel@....com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/7] Support for GB pages in KVM

On Sun, Mar 29, 2009 at 03:03:28PM +0300, Avi Kivity wrote:
> Joerg Roedel wrote:
>> On Sat, Mar 28, 2009 at 06:40:08PM -0300, Marcelo Tosatti wrote:
>>   
>>> On Fri, Mar 27, 2009 at 03:31:52PM +0100, Joerg Roedel wrote:
>>>     
>>>> Hi,
>>>>
>>>> this patchset extends the KVM MMU implementation to support 1GB pages as
>>>> supported by AMD family 16 processors. These patches enable support for
>>>> 1 GB pages with Nested Paging. Support for these pages in the shadow
>>>> paging code was also developed but does not run stable yet. The patch
>>>> for shadow-paging support is not included in this series and will be
>>>> sent out seperatly.
>>>>       
>>> Looks generally sane. I'm not sure its even worthwhile to support
>>> GBpages with softmmu, because the chance of finding an area without
>>> shadowed (write protected) pages is much smaller than with 2MB pages.
>>>     
>>
>> Thanks for your review.
>>
>> The idea behind GB pages in softmmu code was to provide GB pages to the
>> guest even if hardware does not support it. This would work better with
>> live migration (Only case where we wouldn't have gbpages then would be
>> vmx with ept enabled).
>>
>>   
>>> Have any numbers to share?
>>>     
>>
>> No numbers I fully trust by now. I measured a 32% improvement in
>> kernbench using nested pages backed with gb pages. I will do some more
>> measurements and share some more solid numbers.
>>
>>   
>
> Compared to 2M pages?  But we're already close to native here.

Yes, thats why I don't trust those numbers. I'll find out what went
wrong and provide more solid numbers.

	Joerg

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