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Re: [kvm-devel] [Qemu-devel] Re: [PATCH 1/6] Use correct types to enable

From: Anthony Liguori
Subject: Re: [kvm-devel] [Qemu-devel] Re: [PATCH 1/6] Use correct types to enable > 2G support
Date: Fri, 01 Feb 2008 14:31:53 -0600
User-agent: Thunderbird (X11/20071229)

Daniel P. Berrange wrote:
On Fri, Feb 01, 2008 at 11:53:02AM -0600, Anthony Liguori wrote:
Ian Jackson wrote:
Anthony Liguori writes ("[Qemu-devel] Re: [kvm-devel] [PATCH 1/6] Use correct types to 
enable > 2G support"):
The alternative is to change all the places that assume phys_ram_base + PA which I don't like very much.
We would ideally like to do this for Xen, at least in the places we
care about.  (Xen uses less of the qemu tree than KVM, I think.)
Support for the map cache in the Xen tree is a rather big change that I'm not going to attempt to support it in this patch series.

I'd rather preserve the phys_ram_base + PA assumption because it allows us to be able to do support > 1 page DMA operations for our virtual IO drivers. If you break the assumption that physically contiguous memory in the guest is virtual contiguous memory in the host, things get pretty ugly.

Well Xen i386 has no choice but to use the map cache, since PAE lets i386 guests have as much as 100 GB of memory & there's no way you can
map that into QEMU's 32-bit userspace. So if virt IO has a dependancy
on contigious memory access in QEMU its not going to play nice with

For KVM (and it sounds like QEMU), we're just making the statement that 32-bit hosts cannot support > 2GB guests. I know that's a regression for Xen but in all fairness, I did raise this as an objection when the map cache was first introduced :-)

virtio could still be made to work with map cache. You would just have to change it to be able to map more than one page contiguously. As I mentioned though, it just starts getting ugly.


Anthony Liguori


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