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

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Message-ID: <alpine.DEB.2.02.1506231441280.16917@kaball.uk.xensource.com>
Date:	Tue, 23 Jun 2015 14:41:42 +0100
From:	Stefano Stabellini <stefano.stabellini@...citrix.com>
To:	Stefano Stabellini <stefano.stabellini@...citrix.com>
CC:	Julien Grall <julien.grall@...rix.com>,
	David Vrabel <david.vrabel@...rix.com>,
	<xen-devel@...ts.xenproject.org>, <wei.liu2@...rix.com>,
	<ian.campbell@...rix.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

On Tue, 23 Jun 2015, Stefano Stabellini wrote:
> On Mon, 18 May 2015, Julien Grall wrote:
> > 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.
> 
> We could at least introduce a few asserts, so that an ARM64 kernel
> build, that any x86 maintainers can easily and quickly do on their x86
> machines, would spot these errors.

I actually meant BUILD_BUG_ON
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