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

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Message-ID: <555614AB.9000006@citrix.com>
Date:	Fri, 15 May 2015 16:45:47 +0100
From:	David Vrabel <david.vrabel@...rix.com>
To:	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>, <david.vrabel@...rix.com>,
	<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

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?

David
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