Re: Questions about virtio DMA operations

[Peter Maydell]

On Tue, 31 Aug 2021 at 05:48, Arnabjyoti Kalita
<akalita@cs.stonybrook.edu> wrote:
>
> Dear Peter,
>
> I had a few additional questions with regards to dma_memory_map(). So,
> let's say I record the buffer that has been written/read from the
> disk.
>
> iov[num_sg].iov_base = dma_memory_map(vdev->dma_as, pa, &len, s_write ?
>                      DMA_DIRECTION_FROM_DEVICE :
>                      DMA_DIRECTION_TO_DEVICE);
>
> addr[num_sg] = pa;
> iov[num_sg].iov_len = len;
>
> Is it correct to say that the host virtual address(in the case above,
> iov[num_sg].iov_base) that dma_memory_map gives will contain the
> actual buffer that has been read/written from/to the device?

It contains a pointer which corresponds to the guest memory.
If this is a read-from-guest-memory, then the contents will
be whatever the guest just asked us to do IO from. (Whether
we actually end up pushing that to the block device depends on
a bunch of other stuff, I expect.)

If this is a write-to-guest-memory, then at the point where
dma_memory_map() has just been called, then the contents are
whatever random junk is currently at that memory. The virtio
device model will at some point arrange for the block layer
to do IO into that host address.

> And when dma_memory_unmap() happens, the contents of the buffer that
> is pointed to by the host virtual address, will be finally written to
> the guest physical memory which is pointed to by "pa" in the code
> above?

If the DMA is to real guest RAM, then the IO will update the
guest memory immediately as the block layer writes to it,
because the iov_base pointer is really to the actual host memory
that is backing that guest RAM. It's only if the DMA is to something
that's a device model (not RAM) that the write is delayed until
the unmap call.

-- PMM