Date: Fri, 8 Jan 2010 11:13:29 -0500 From: John Baldwin <jhb@freebsd.org> To: Mark Tinguely <tinguely@casselton.net> Cc: freebsd-hackers@freebsd.org Subject: Re: bus_dmamap_load_uio() and user data Message-ID: <201001081113.30008.jhb@freebsd.org> In-Reply-To: <201001081414.o08EEaBM053148@casselton.net> References: <201001081414.o08EEaBM053148@casselton.net>
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On Friday 08 January 2010 9:14:36 am Mark Tinguely wrote: > > You should use the pmap from the thread in the uio structure. Similar to > > this from the x86 bus_dma code: > > > > if (uio->uio_segflg == UIO_USERSPACE) { > > KASSERT(uio->uio_td != NULL, > > ("bus_dmamap_load_uio: USERSPACE but no proc")); > > pmap = vmspace_pmap(uio->uio_td->td_proc->p_vmspace); > > } else > > pmap = NULL; > > > > Later when doing VA -> PA conversions the code does this: > > > > if (pmap) > > paddr = pmap_extract(pmap, vaddr); > > else > > paddr = pmap_kextract(vaddr); > > > > We do that, but I notice that all the architecture that implement > bounce buffers assume the VA is in the current map. Most of the > addresses are KVA, but bus_dmamap_load_uio() can be in the user space. > > I was wondering about the sequence: > > bus_dmamap_load_uio() user space > dma_load_buffer() > add bounce page save UVA (in caller user map) > > later: > > bus_dma_sync > copies bounce buffer from saved UVA. <- here is my concern. The user pmap > is not remembered use current pmap. > > Since the bounce buffer copy routines have been running in other architectures > for years without corruption, I was wondering we can safely assume that the > dma sync is running in the same thread/address space as the bus_dmamap_load_uio > call. I was hoping you would say, don't worry the scheduler would always > reload the same thread to execute the dma sync code ... Ahh. I think bus_dmamap_load_uio() doesn't do deferred callbacks (i.e. mandates BUS_DMA_NOWAIT), and probably is always invoked from curthread. Even in the case of aio, the thread's vmspace is the effective one at the time bus_dmamap_load_uio() would be invoked, so in practice it is safe. -- John Baldwin
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