on 10/01/2012 22:54 Matthew Jacob said the following: > > I think it would be just simpler to disallow page zero usage period.
That would be simpler indeed. > Can you > think of any case where physical page 0 is ever a valid DMA address? Not sure if I got your question right. I think that it's possible that an arbitrary virtual page with data could reside at the physical address zero. One particular case that I ran into in practice is that at least amd64 dump code always includes page zero into a dump and the umass+usb code tries to do a direct dma from it. > At the very least, require bounce buffers. Not sure if I got this suggestion in this terse form. Could you please explain? Thank you! > On Tue, 10 Jan 2012, Andriy Gapon wrote: > >> >> >> Some hardware interfaces may reserve a special meaning for a (physical) >> memory >> address value of zero. One example is the OHCI specification where a zero >> value >> in CurrentBufferPointer doesn't mean a physical address, but has a reserved >> meaning. To be honest I don't have another example :) but don't preclude its >> existence. >> >> To deal with this peculiarity we could use a special flag/quirk that would >> instruct the bus dma code to never use the page zero for communication with >> the >> hardware. >> Here's a proof of concept patch that implements the idea: >> http://people.freebsd.org/~avg/usb-dma-pagezero.diff >> >> Some concerns: >> - not sure if BUS_DMA_NO_PAGEZERO is the best name for the flag >> - the patch implements the flag only for x86 at the moment >> - usb code uses the flag regardless of the actual controller type >> >> What do you think? >> >> -- >> Andriy Gapon >> _______________________________________________ >> freebsd-current@freebsd.org mailing list >> http://lists.freebsd.org/mailman/listinfo/freebsd-current >> To unsubscribe, send any mail to "freebsd-current-unsubscr...@freebsd.org" >> -- Andriy Gapon _______________________________________________ freebsd-current@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-current To unsubscribe, send any mail to "freebsd-current-unsubscr...@freebsd.org"
