[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] [Xen-devel] Re: [Patch] by default don't give all memory to dom0
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 systemswith 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 _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxxxxxxxx http://lists.xensource.com/xen-devel
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