Re: [Xen-devel] Question about DMA on 1:1 mapping dom0 of arm64

[Chen Baozi <cbz@xxxxxxxxxx>]

On Fri, Apr 17, 2015 at 05:13:16PM +0100, Stefano Stabellini wrote:
> On Fri, 17 Apr 2015, Ian Campbell wrote:
> > On Fri, 2015-04-17 at 15:34 +0100, Stefano Stabellini wrote:
> > > > > If I set dom0_mem to a small value (e.g. 512M), which makes all 
> > > > > physical memory
> > > > > of dom0 below 4G, everything goes fine.
> > > > 
> > > > So you are getting allocated memory below 4G?
> > > > 
> > > > You message on IRC suggested you weren't, did you hack around this?
> > > > 
> > > > I think we have two options, either xen_swiotlb_init allocates pages
> > > > below 4GB (e.g. __GFP_DMA) or we do something to allow xen_swiotlb_fixup
> > > > to actually work even on a 1:1 dom0.
> > > 
> > > I don't think that making xen_swiotlb_fixup work on ARM is a good idea:
> > > it would break the 1:1.
> > 
> > This would actually work though, I think, because this is the swiotlb so
> > we definitely have the opportunity to return the actual DMA address
> > whenever we use this buffer and the device will use it in the right
> > places for sure.
> 
> The code is pretty complex as is -- I would rather avoid adding more
> complexity to it.  For example we would need to bring back a mechanism
> to track dma address -> pseudo-physical address mappings on arm, even
> though it would be far simpler of course.
> 
> Also I think it makes sense to use the swiotlb buffer for its original
> purpose.
> 
> If we could introduce a mechanism to get a lower than 4G buffer in dom0,
> but matching the 1:1, I think it would make the maintenance much easier
> on the linux side.

+1

Actually, we have already had the mechanism on arm32 to populate at least
one bank of memory below 4G. Thus, the only thing we have to do on the
hypervisor side is to make arm32 and arm64 share the same process in
allocate_memory_11(), removing the 'lowmem = is_32bit_domain(d)' related
conditions. If this is acceptable, the only thing we need to do in Linux
kernel is to add the __GFP_DMA flag when allocating pages for xen_io_tlb_start
in xen_swiotlb_init.

Baozi.

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