Re: [RFC PATCH v2] Utilize the PCI API in the TTM framework.

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

On Wed, Jan 12, 2011 at 10:12:14AM +0100, Thomas Hellstrom wrote:
> Hi, Konrad.
> 
> This discussion has become a bit lenghty. I'll filter out the
> sorted-out stuff, which leaves me with two remaining issues:

<nods>
> 
> 
> On 01/11/2011 04:55 PM, Konrad Rzeszutek Wilk wrote:
> >
> >So at the end we have 16GB taken from 8GB->24GB, and 320MB taken from
> >0->4GB. When you start allocating coherent memory from each guest
> >(and yeah, say we use 2GB each), we end up with the first guest getting
> >the 2GB, the second getting 1.7GB, and then the next two getting zil.
> >
> >You still have GFP_KERNEL memory in each guest - the first one has 2GB left
> >, then second 2.3, the next two have each 4GB.
> >
> >> From the hyprevisor pool perspective, the 0-4GB zone is exhausted, so
> >is the 8GB->24GB, but it still has 4GB->8GB free - so it can launch one more
> >guest (but without PCI passthrough devices).
> >
> >>On a 4GB machine or less, that would be the same as kernel memory.
> >>Now, if 4 guests think they can allocate 2GB of coherent memory
> >>each, you might run out of kernel memory on the host?
> >So host in this case refers to the Hypervisor and it does not care
> >about the DMA or what - it does not have any device drivers(*) or such.
> >The first guest (dom0) is the one that deals with the device drivers.
> >
> >*: It has one: the serial port, but that is not really that important
> >for this discussion.
> 
> Let's assume we're at where the hypervisor (or host) has exhausted
> the 0-4GB zone, due to guests coherent memory allocations, and that
> the physical machine has 4GB of memory, all in the 0-4GB zone. Now
> if the hypervisor was running on a Linux kernel, there would be no
> more GFP_KERNEL memory available on the *host* (hypervisor), and the
> hypervisor would crash. Now I don't know much about Xen, but it
> might be that this is not a problem with Xen at all?

It will have no problem. It allocates at boot all the memory it needs
and won't get bigger (or smaller) after that.

> 
> 
> >>
> >>Another thing that I was thinking of is what happens if you have a
> >>huge gart and allocate a lot of coherent memory. Could that
> >>potentially exhaust IOMMU resources?
> ><scratches his head>
> 
> I need to be more specific. Let's assume we're on "bare metal", and
> we want to allocate 4GB of coherent memory. For most IOMMUs that
> would mean as you previously state, that we actually allocate
> GFP_DMA32 memory. But for some IOMMUs that would perhaps mean that
> we allocate *any* memory and set up a permanent DMA mapping in the
> IOMMU for the coherent pages. What if, in such a case, the IOMMU can
> only set up 2GB of coherent memory?
> 
> Or in short, can there *ever* be "bare metal" cases where the amount
> of coherent memory available is less than DMA32 memory available?

There is no such case where the amount of coherent memory is
less than DMA32 memory. [unless the IOMMU has some chipset problem where it can't map
2^31 -> 2^32 addresses, but that is not a something we should worry
about]