Re: How to allocate contiguous RAM in pv guests

[Uli <um@amd64.org>]

On Tue 30.01.07 05:28, Guy Zana wrote:
> Hi,
> 
> An idea is to scan the bios e820 table and split a large block to reserve those 64MB.
> Than, you can mark this block as E820_GART or something and add your own allocator next to the xen-heap allocator.
> To map it contigiously, you can use the fixmap from the I/O remapping area.
> Is this what you need?

Yeah, I would have needed something along these lines.
However, I just found out that there's an adaptive mechanism due to
which I don't need to implement anything at all.

Xen will (unless dom0_mem is provided) keep 1/16th of the memory free
and in the heap. For a >4GB system, this is at least 256mb, leaving
enough memory for the aperture. This mechanism is also the reason why I
experienced a case where there wasn't enough memory...that was on a
system with 1GB memory, 1/16th being 64MB. Since the GART is usually
deactivated on systems with less than 4GB RAM, there is no problem at
all.

I'm relieved :)
Thanks for your help,

Uli

> 
> Thanks,
> Guy. 
> 
> > -----Original Message-----
> > From: xen-devel-bounces@lists.xensource.com 
> > [mailto:xen-devel-bounces@lists.xensource.com] On Behalf Of Uli
> > Sent: Monday, January 29, 2007 4:55 PM
> > To: xen-devel@lists.xensource.com
> > Subject: [Xen-devel] How to allocate contiguous RAM in pv guests
> > 
> > Hi!
> > 
> > I'm working on a patch to get the GART as an IOMMU working in 
> > linux/dom0.
> > However, the problem I describe below applies equally to the 
> > software IOMMU.
> > 
> > If the BIOS doesn't setup the aperture it has to be allocated 
> > from memory. Therefore, one needs a contiguous memory region, 
> > currently 64MB.
> > The software IOMMU always needs a contiguous memory region 
> > (same size).
> > 
> > In order to do this, the hypercall XENMEM_exchange is given a 
> > bunch of mfns and returns a (host physically) contiguous 
> > memory region.
> > Unfortunately, the implementation allocates the contiguous 
> > memory from the heap first and then returns the discontiguous 
> > mfns to it. Therefore, there has to be a (in this case) 64MB 
> > chunk of memory in the xen heap available for the call to succeed.
> > I have observed that on most machines exactly one such chunk 
> > is available. However, I've also had a machine where this is 
> > not the case.
> > 
> > It seems to me that using the xen heap is not the right thing to do.
> > The only other option I can think of is scanning dom0's 
> > memory for a (host physical) chunk of memory that
> > a) belongs entirely to it and
> > b) is free
> > 
> > Once such a chunk is found, one would have to map it 
> > contiguously into virtual memory.  Actually, the latter is 
> > only neccessary for the software IOMMU. The GART aperture 
> > doesn't have to be in virtual memory since it is only 
> > accessed from devices.
> > 
> > Thanks for any suggestions,
> > 
> > Uli
> > 
> > 
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> > 
>