[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [Xen-users] ARM: "xen_add_mach_to_phys_entry: cannot add ... already exists and panics"
On 04/07/14 15:31, Stefano Stabellini wrote: > On Fri, 4 Jul 2014, David Vrabel wrote: >> On 04/07/14 15:12, Stefano Stabellini wrote: >>> On Fri, 4 Jul 2014, David Vrabel wrote: >>>> On 03/07/14 18:47, Stefano Stabellini wrote: >>>>> >>>>> At the moment I would like a way to disable grant mappings and go back >>>>> to grant copies on demand. Maybe we could have a feature flag to change >>>>> the behaviour of the network backend? >>>> >>>> You must fix the ARM code to handle this properly. >>>> >>>> blkback also uses grant maps and depending on the frontend blkback may >>>> grant map the same machine frame multiple time. We have see frontends >>>> that send such requests. >>> >>> Indeed, it must be fixed properly. The workaround of disabling grant >>> mappings would be just to buy us some time to come up with the fix. >> >> It's an expensive workaround though. > > In terms of performances or in terms of code? > If it's the performances you are worried about, we could enable the > workaround just for arm and arm64. Expensive in terms of effort required to implement and maintain. >>>> I can't remember how the ARM code works. Where is the problematic m2p >>>> lookup? >>> >>> arch/arm/xen/p2m.c >>> >>> >>>> Perhaps this could be avoided for foreign frames? >>> >>> Unfortunately no. The whole point of p2m.c is to be able to translate >>> foreign frames for dma operations. >> >> This is a p2m lookup though, which is fine, yes? Where, specifically is >> a mfn_to_pfn() lookup needed for a foreign MFN. > > drivers/xen/swiotlb-xen.c:xen_unmap_single. xen_bus_to_phys is based on > the value returned by mfn_to_pfn. I think you can probably get away with not doing the cache operations on foreign pages when DMA map/unmapping. DMA mapped foreign pages are (currently) either: a) packets from a guest being Tx'd off host. These are read-only and are immediately freed after they're tranmitted. b) block requests from blkback and these pages are never accessed. David _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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