On Thu, Nov 22, 2012 at 10:34:13AM +0100, Paolo Bonzini wrote: > Il 21/11/2012 19:32, Stefan Hajnoczi ha scritto: > > The iov_get_ptr() data returns a pointer to contiguous data within a > > vector. This allows the caller to manipulate data inside the vector > > without copying in/out using iov_from_buf()/iov_to_buf() when we know > > that data is contiguous within an iovec element. > > This works for you because you have a single byte to write. It would > not work for the SG_IO inhdr, which would need iov_to_buf().
Guilty as charged, your honor. :) Let me give a few more details about the motivation for this function: In virtio-blk-data-plane we have an iovec[] array. In the read/write code path we discard the inhdr/outhdr so just the data buffers are left in the iovec[] array. Then we can pass the iovec[] array straight to the Linux AIO functions. Because we're using the iovec[] array for data buffers and we're not allowed to make assumptions about iovec layout, we cannot use iov_to_buf()/iov_from_buf() at the end to fill in the status field - the inhdr has already been discarded from the iovec[] array. Since I knew the inhdr is only 1 byte I decided against doing something like dynamically allocating/freeing a QEMUIOVector which could handle spanning iovecs. That said, I think this function is okay as-is because it works fine for non-virtio cases where the caller *knows* the iovec[] layout. As a utility function it stands on its own. > What about the following alternative API: > > void *iov_get_ptr(struct iovec *iov, unsigned int iov_cnt, > ssize_t offset, size_t *bytes); > > which would place the number of valid bytes (i.e. the length of the > remainder of the iovec entry) in *bytes? > > Also, I think that offset == iov_size(iov, iov_cnt) should be > acceptable, and it would be the only case in which *bytes == 0. Hmm...this may be more useful than the version I proposed since the caller can also use it to find out how many bytes are contiguous. Michael: Any concerns if I update the code to reflect Paolo's suggestion? Stefan
