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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