linux-kernel - Re: [PATCH v4 00/20] xen/arm64: Add support for 64KB page in Linux

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Message-ID: <55F6ADAE.1010609@citrix.com>
Date:	Mon, 14 Sep 2015 12:21:18 +0100
From:	Julien Grall <julien.grall@...rix.com>
To:	Roger Pau Monné <roger.pau@...rix.com>,
	<xen-devel@...ts.xenproject.org>
CC:	<wei.liu2@...rix.com>, <ian.campbell@...rix.com>,
	<konrad.wilk@...cle.com>, <stefano.stabellini@...citrix.com>,
	<linux-kernel@...r.kernel.org>, <david.vrabel@...rix.com>,
	<boris.ostrovsky@...cle.com>,
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v4 00/20] xen/arm64: Add support for 64KB page in Linux

On 14/09/15 12:04, Roger Pau Monné wrote:
> Hello,
> 
> El 14/09/15 a les 12.40, Julien Grall ha escrit:
>> Hi Roger,
>>
>> On 14/09/15 09:56, Roger Pau Monné wrote:
>>> El 07/09/15 a les 17.33, Julien Grall ha escrit:
>>>> Hi all,
>>>>
>>>> ARM64 Linux is supporting both 4KB and 64KB page granularity. Although, Xen
>>>> hypercall interface and PV protocol are always based on 4KB page granularity.
>>>>
>>>> Any attempt to boot a Linux guest with 64KB pages enabled will result to a
>>>> guest crash.
>>>>
>>>> This series is a first attempt to allow those Linux running with the current
>>>> hypercall interface and PV protocol.
>>>>
>>>> This solution has been chosen because we want to run Linux 64KB in released
>>>> Xen ARM version or/and platform using an old version of Linux DOM0.
>>>>
>>>> There is room for improvement, such as support of 64KB grant, modification
>>>> of PV protocol to support different page size... They will be explored in a
>>>> separate patch series later.
>>>>
>>>> TODO list:
>>>>     - Convert swiotlb to 64KB
>>>>     - Convert xenfb to 64KB
>>>>     - Support for multiple page ring support
>>>>     - Support for 64KB in gnttdev
>>>>     - Support of non-indirect grant with 64KB frontend
>>>>     - It may be possible to move some common define between
>>>>     netback/netfront and blkfront/blkback in an header
>>>>
>>>> I've got most of the patches for the TODO items. I'm planning to send them as
>>>> a follow-up as it's not a requirement for a basic guests.
>>>>
>>>> All patches has been built tested for ARM32, ARM64, x86. But I haven't tested
>>>> to run it on x86 as I don't have a box with Xen x86 running. I would be
>>>> happy if someone give a try and see possible regression for x86.
>>>
>>> Do you have figures regarding if/how much performance penalty do the
>>> blkfront/blkback patches add to the traditional 4KB scenario (frontend
>>> and backend running on guests using 4KB pages)?
>>
>> Which benchmark do you advice? Although, I don't have SSD on this
>> platform so it may be possible that the bottleneck will be the hard drive.
> 
> I've normally used a ramdisk in order to test performance, but it seems
> nullblk would be a better option (it wasn't available before) and it
> doesn't reduce the size of RAM available to the system:
> 
> https://www.kernel.org/doc/Documentation/block/null_blk.txt

I will give a look to this.

>>>
>>> Since there's been no design document about this and the TODO list
>>> doesn't contain it, I would like to know which plans do we have in order
>>> to fix this properly.
>>
>> Can you explain what kind of design document you were expecting? The
>> support of 64KB page granularity is pretty straight-forward and there is
>> not many way to do it. We have to split the page in 4KB chunk to feed
>> the ring.
> 
> I was thinking about adding support for 64KB grants with the aim of
> eventually removing the splitting done in xen-blkfront and the
> grant-table code in general.

I'd like to see a basic support of 64KB page granularity upstream before
starting to think about performance improvement. And there is still a
lot to do.

Although, having 64KB grants won't remove the splitting as we would
still have to support frontend/backend which only handle 4KB grant.


>> TBH, I'm expecting a small impact to the performance. It would be hard
>> to get the exactly the same performance as today if we keep the helpers
>> to avoid the backend dealing himself with the splitting and page
>> granularity.
>>
>> Although, if the performance impact is not acceptable, it may be
>> possible to optimize gnttab_foreach_grant_in_range by moving the
>> function inline. The current way to the loop is the fastest I've found
>> (I've wrote a small program to test different way) and we will need it
>> when different of size will be supported.
> 
> I don't expect the performance to drop massively with this patches
> applied, but it would be good to al least have an idea of the impact.

I will only be able to give percentage. I guess this would be sufficient
for you?

Regards,

-- 
Julien Grall
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