Re: [kvm-devel] [Qemu-devel] Re: [PATCH 1/6] Use correct types to enable 2G support
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. Regards, Anthony Liguori In Xen, the guest memory is not in general mapped into the host qemu's address space. Ian. - This SF.net email is sponsored by: Microsoft Defy all challenges. Microsoft(R) Visual Studio 2008. http://clk.atdmt.com/MRT/go/vse012070mrt/direct/01/ ___ kvm-devel mailing list [EMAIL PROTECTED] https://lists.sourceforge.net/lists/listinfo/kvm-devel
Re: [kvm-devel] [Qemu-devel] Re: [PATCH 1/6] Use correct types to enable 2G support
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 Xen. Dan. -- |=- Red Hat, Engineering, Emerging Technologies, Boston. +1 978 392 2496 -=| |=- Perl modules: http://search.cpan.org/~danberr/ -=| |=- Projects: http://freshmeat.net/~danielpb/ -=| |=- GnuPG: 7D3B9505 F3C9 553F A1DA 4AC2 5648 23C1 B3DF F742 7D3B 9505 -=|
Re: [kvm-devel] [Qemu-devel] Re: [PATCH 1/6] Use correct types to enable 2G support
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 Xen. 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. Regards, Anthony Liguori Dan.
Re: [kvm-devel] [Qemu-devel] Re: [PATCH 1/6] Use correct types to enable 2G support
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. That's why you should be using the cpu_physical_memory_rw routines :-) Anything that assume large linear accesses (Currently only some of the embedded LCD controllers) is going to break as soon as you start introducing IOMMUs. There have been several threads on this list about having a sane DMA infrastructure. Paul
