[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:

quoted

Hi David,

On 15/05/15 16:45, David Vrabel wrote:

quoted

On 14/05/15 18:00, Julien Grall wrote:

quoted

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