Re: Xen patches

[Manuel Bouyer <bouyer%antioche.eu.org@localhost>]

On Tue, Sep 22, 2015 at 08:58:21AM +0530, Cherry G. Mathew wrote:
> Hi,
> 
> Here's a bunch of drm patches for dom0. Works on my ivybridge with
> dom0_mem=2046M
> 
> They will *not* work if you allocate more ram than your h/w dma can
> address.

I think it's worse than that: it won't work if you have more physical RAM
than your HW DMA can address. It may work at boot time but not later.
There is no guarantee that your dom0 has memory in the low addresses.
In fact, during the dom0 life there will be machine pages deallocated and
reallocated (in the driver backends for example), and these pages
may be in high memory.

For this to work, pages with PA constraints should be allocated by the
Xen backend, which can request memory from the hypervisor with the right
addresses. But for this the whole stuff with freelist has to be redone
to get memory from a port-specific allocator instead of freelists
and this probably requires changes to UVM, and better integration with
bus_dma(9) (the fundamental problem being that the drm code does DMA without
using bus_dma(9)). This will help other non-x86 ports too.
I don't want to be rude (and I'm glad you're working on this) but I think
your patch goes in the wrong direction.

-- 
Manuel Bouyer <bouyer%antioche.eu.org@localhost>
     NetBSD: 26 ans d'experience feront toujours la difference
--