[Top][All Lists]
[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: [Qemu-devel] [PATCH v3 09/12] iov: add iov_get_ptr() to reference ve
From: |
Michael S. Tsirkin |
Subject: |
Re: [Qemu-devel] [PATCH v3 09/12] iov: add iov_get_ptr() to reference vector data |
Date: |
Thu, 22 Nov 2012 14:35:46 +0200 |
On Thu, Nov 22, 2012 at 12:58:23PM +0100, Stefan Hajnoczi wrote:
> 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.
How about using iov_copy?
We have exactly this problem in virtio net if we run
on host that does not support mergeable buffer header,
and we solve it by copying out the iovec.
> 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.
>
My concern is these APIs are unsafe to use: you get back a pointer and
you must verify length is not too big before access. Since the iov can
be manipulated by guest this looks like a good place to put extra
safeguards.
> > 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
I'd prefer something that actually works for all cases
rather than making callers check and handle failure,
or reason why it can't fail.
--
MST
- [Qemu-devel] [PATCH v3 10/12] test-iov: add iov_get_ptr() test case, (continued)
- [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, 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 <=
- 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