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Re: [Qemu-devel] [PATCH v3 09/12] iov: add iov_get_ptr() to reference ve
From: |
Stefan Hajnoczi |
Subject: |
Re: [Qemu-devel] [PATCH v3 09/12] iov: add iov_get_ptr() to reference vector data |
Date: |
Thu, 22 Nov 2012 12:58:23 +0100 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
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
- [Qemu-devel] [PATCH v3 07/12] iov: add iov_discard() to remove data, (continued)
- [Qemu-devel] [PATCH v3 07/12] iov: add iov_discard() to remove data, Stefan Hajnoczi, 2012/11/21
- [Qemu-devel] [PATCH v3 02/12] configure: add CONFIG_VIRTIO_BLK_DATA_PLANE, Stefan Hajnoczi, 2012/11/21
- [Qemu-devel] [PATCH v3 10/12] test-iov: add iov_get_ptr() test case, Stefan Hajnoczi, 2012/11/21
- [Qemu-devel] [PATCH v3 09/12] iov: add iov_get_ptr() to reference vector data, Stefan Hajnoczi, 2012/11/21
- Re: [Qemu-devel] [PATCH v3 09/12] iov: add iov_get_ptr() to reference vector data, Paolo Bonzini, 2012/11/22
- Re: [Qemu-devel] [PATCH v3 09/12] iov: add iov_get_ptr() to reference vector data, Michael S. Tsirkin, 2012/11/22
- Re: [Qemu-devel] [PATCH v3 09/12] iov: add iov_get_ptr() to reference vector data, Paolo Bonzini, 2012/11/22
- Re: [Qemu-devel] [PATCH v3 09/12] iov: add iov_get_ptr() to reference vector data, Michael S. Tsirkin, 2012/11/22
- Re: [Qemu-devel] [PATCH v3 09/12] iov: add iov_get_ptr() to reference vector data, Paolo Bonzini, 2012/11/22
- Re: [Qemu-devel] [PATCH v3 09/12] iov: add iov_get_ptr() to reference vector data, Michael S. Tsirkin, 2012/11/22
- Re: [Qemu-devel] [PATCH v3 09/12] iov: add iov_get_ptr() to reference vector data,
Stefan Hajnoczi <=
- Re: [Qemu-devel] [PATCH v3 09/12] iov: add iov_get_ptr() to reference vector data, Paolo Bonzini, 2012/11/22
- Re: [Qemu-devel] [PATCH v3 09/12] iov: add iov_get_ptr() to reference vector data, Michael S. Tsirkin, 2012/11/22
- Re: [Qemu-devel] [PATCH v3 09/12] iov: add iov_get_ptr() to reference vector data, Stefan Hajnoczi, 2012/11/22
[Qemu-devel] [PATCH v3 12/12] virtio-blk: add x-data-plane=on|off performance feature, Stefan Hajnoczi, 2012/11/21
[Qemu-devel] [PATCH v3 11/12] dataplane: add virtio-blk data plane code, Stefan Hajnoczi, 2012/11/21
[Qemu-devel] [PATCH v3 08/12] test-iov: add iov_discard() testcase, Stefan Hajnoczi, 2012/11/21
[Qemu-devel] [PATCH v3 03/12] dataplane: add host memory mapping code, Stefan Hajnoczi, 2012/11/21
[Qemu-devel] [PATCH v3 04/12] dataplane: add virtqueue vring code, Stefan Hajnoczi, 2012/11/21