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

[Roger Pau MonnÃ]

Hello,

El 14/09/15 a les 12.40, Julien Grall ha escrit:
> Hi Roger,
> 
> On 14/09/15 09:56, Roger Pau Monnà 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

>>
>> 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.

Some modifications would be needed in xen-blkback in order to support
mapping 64KB grants, but this could also be used by x86 if we ever
enable 2MB grants and allow them to be used in the block PV protocol,
while the current code can only be used by ARM.

> 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.

Roger.

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