[Xen-devel] Re: [Patch] by default don't give all memory to dom0

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 18 Aug 2005, at 20:24, Siddha, Suresh B wrote:

> By default, xen needs to reserve some portion of memory to satisfy
> SWIOTLB and other contguous memory region requests. Following
> the current swiotlb enabling mechanism, Appended patch reserves 128MB
> of memory on systems with more than 2GB of RAM.
Hmmm... sounds reasonable. I'd rather have one dom0 memory parameter 
though --- keeping dom0_mem but have +ve value mean 'allocate this 
amount' and -ve mean 'allocate full memory - this amount'. Otherwise we 
have two competing parameters specifying basically the same thing...
I don't much like hacky 'policies' that hardcode default reservations 
in the hypervisor, but I think this one is pretty sensible.
> Ideally shouldn't we enable SWIOTLB in dom0 and this DMA memory 
> reservation
> in hypervisor by default? Otherwise we will have a problem(even on 
> systems
> with less than 2GB of RAM) in servicing a driver DMA request to a
> kmalloc'd buffer which spans more than a page or the various
> xen_create_contiguous_region() requests.
You get a pretty clear BUG() message out if this happens. It actually 
tells you to enable 'swiotlb=force'!  The number of drivers that 
actually use multi-page buffers is really small -- we only fixed that 
case in 2.0 a few weeks ago.
I'd rather not waste 64MB of pre-reserved bounce buffers on 
small-memory systems that almost certainly don't need bounce buffers.
 -- Keir


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