linux-kernel - Re: [PATCH] virtio_console: Add support for multiple ports for generic guest and host communication

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Date:	Thu, 17 Sep 2009 22:16:33 +0530
From:	Amit Shah <amit.shah@...hat.com>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	Markus Armbruster <armbru@...hat.com>,
	Anthony Liguori <anthony@...emonkey.ws>,
	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org, greg@...ah.com
Subject: Re: [PATCH] virtio_console: Add support for multiple ports for
	generic guest and host communication

On (Thu) Sep 17 2009 [16:57:01], Alan Cox wrote:
> > Alan, I'm not sure how many ports at a time people would want to use so
> > allocating one major device for this seems OK?
> 
> We have very large minor number ranges now so one dynamic major should do
> you for a while yet. Probably forever but thats always asking for a
> "640K.." moment ;)

:-) OK.

> > +static ssize_t fill_readbuf(struct virtio_console_port *port,
> > +			    char *out_buf, size_t out_count, bool to_user)
> > +{
> 
> Save yourself serious grief - and the FIXME pain noted below - unless you
> are shipping huge objects use a bounce buffer and kill off the to_user
> stuff - modern CPUs are so fast doing cache transfers it really isn't
> worth the suffering for small stuff.

I would if I knew what it is that you are suggesting. A bounce buffer
for guest kernel - guest userspace communication?

This is what it looks like right now:

For guest kernel - host communication, I'm using virtio - which is a
bounce buffer (guest allocates pages, host fetches, reads/writes, guest
gets an ack).

But I probably didn't get what you meant, so please explain.

> > +			ret = copy_to_user(out_buf + out_offset,
> > +					   buf->buf + buf->offset,
> > +					   copy_size);
> > +			/* FIXME: Deal with ret != 0 */

		Amit
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