linux-kernel - Re: [Xen-devel] [RFC 00/23] arm64: Add support for 64KB page granularity in Xen guest

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Date:	Mon, 18 May 2015 13:23:34 +0100
From:	Julien Grall <julien.grall@...rix.com>
To:	David Vrabel <david.vrabel@...rix.com>,
	Julien Grall <julien.grall@...rix.com>,
	<xen-devel@...ts.xenproject.org>
CC:	<wei.liu2@...rix.com>, <ian.campbell@...rix.com>,
	<stefano.stabellini@...citrix.com>, <tim@....org>,
	<linux-kernel@...r.kernel.org>, <boris.ostrovsky@...cle.com>,
	<linux-arm-kernel@...ts.infradead.org>, <roger.pau@...rix.com>
Subject: Re: [Xen-devel] [RFC 00/23] arm64: Add support for 64KB page granularity
 in Xen guest

Hi David,

On 15/05/15 16:45, David Vrabel wrote:
> On 14/05/15 18:00, Julien Grall wrote:
>> 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.
> 
> The key problem I see with this approach is the confusion between guest
> page size and Xen page size.  This is going to be particularly
> problematic since the majority of development/usage will remain on x86
> where PAGE_SIZE == XEN_PAGE_SIZE.
> 
> I think it would be nice to keep XEN_PAGE_SIZE etc out of front and
> backend drivers.  Perhaps with a suitable set of helper functions?

Even with the helpers, we are not protected from any change in the
frontend/backend that will impact 64K. It won't be possible to remove
all the XEN_PAGE_* usage (there is a lots of places where adding helpers
would not be possible) and we would still have to carefully review any
changes.

I think it may be possible to move the grant table splitting in helpers
which would be helpful to support different grant size.

Although, it would require a big amount of work at least in blkfront.

Regards,

-- 
Julien Grall
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