Re: [PATCH] tools/kvm/9p: Add encode/decode routines for protocol data

Sasha Levin
2011-06-24T23:26:53Z

On Fri, 2011-06-24 at 21:30 +0530, Aneesh Kumar K.V wrote:
> On Fri, 24 Jun 2011 09:47:18 -0400, Sasha Levin <levinsasha928@gmail.com> wrote:
> > On Tue, 2011-06-21 at 15:50 +0530, Aneesh Kumar K.V wrote:
> > > +static void virtio_p9_pdu_read(struct p9_pdu *pdu, void *data, size_t size)
> > > +{
> > > + size_t len;
> > > + int i, copied = 0;
> > > + u16 iov_cnt = pdu->out_iov_cnt;
> > > + size_t offset = pdu->read_offset;
> > > + struct iovec *iov = pdu->out_iov;
> > > +
> > > + for (i = 0; i < iov_cnt && size; i++) {
> > > + if (offset >= iov[i].iov_len) {
> > > + offset -= iov[i].iov_len;
> > > + continue;
> > > + } else {
> > > + len = MIN(iov[i].iov_len - offset, size);
> > > + memcpy(data, iov[i].iov_base + offset, len);
> > > + size -= len;
> > > + data += len;
> > > + offset = 0;
> > > + copied += len;
> > > + }
> > > + }
> > > + pdu->read_offset += copied;
> > > +}
> > > +
> > > +static void virtio_p9_pdu_write(struct p9_pdu *pdu,
> > > + const void *data, size_t size)
> > > +{
> > > + size_t len;
> > > + int i, copied = 0;
> > > + u16 iov_cnt = pdu->in_iov_cnt;
> > > + size_t offset = pdu->write_offset;
> > > + struct iovec *iov = pdu->in_iov;
> > > +
> > > + for (i = 0; i < iov_cnt && size; i++) {
> > > + if (offset >= iov[i].iov_len) {
> > > + offset -= iov[i].iov_len;
> > > + continue;
> > > + } else {
> > > + len = MIN(iov[i].iov_len - offset, size);
> > > + memcpy(iov[i].iov_base + offset, data, len);
> > > + size -= len;
> > > + data += len;
> > > + offset = 0;
> > > + copied += len;
> > > + }
> > > + }
> > > + pdu->write_offset += copied;
> > > +}
> > > +
> >
> > Just wondering here, can we use pipes (read/write/vmsplice) instead of a
> > scatter-gather list?
>
> Can you elaborate on this. The function is to encode/decode data out of
> protocol buffer.

I was just thinking about storing the data in a pipe instead of a
scatter-gather list.

This would turn virtio_p9_pdu_read() into a simple wrapper for read()
from a pipe and virtio_p9_pdu_write() into a wrapper for write().

Something like this:

void virtio_9p__init(struct kvm *kvm, const char *root, const char
*tag_name)
{
[...]
pipe(p9_pipe);
[...]
}

static void virtio_p9_pdu_read(struct p9_pdu *pdu, void *data, size_t
size)
{
read(p9_pipe[0], data, size);
}

static void virtio_p9_pdu_write(struct p9_pdu *pdu, const void *data,
size_t size)
{
write(p9_pipe[1], data, size);
}

--

Sasha.