linux-kernel - Re: [PATCH 1/3] virtio: Basic implementation of virtio pstore driver

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Date:	Mon, 18 Jul 2016 17:29:55 +0900
From:	Namhyung Kim <namhyung@...nel.org>
To:	Cornelia Huck <cornelia.huck@...ibm.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Radim Kr??m???? <rkrcmar@...hat.com>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Anthony Liguori <aliguori@...zon.com>,
	Anton Vorontsov <anton@...msg.org>,
	Colin Cross <ccross@...roid.com>,
	Kees Cook <keescook@...omium.org>,
	Tony Luck <tony.luck@...el.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...nel.org>,
	Minchan Kim <minchan@...nel.org>, kvm@...r.kernel.org,
	qemu-devel@...gnu.org, virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH 1/3] virtio: Basic implementation of virtio pstore driver

Hello,

On Mon, Jul 18, 2016 at 09:54:39AM +0200, Cornelia Huck wrote:
> On Mon, 18 Jul 2016 13:37:39 +0900
> Namhyung Kim <namhyung@...nel.org> wrote:
> 
> > The virtio pstore driver provides interface to the pstore subsystem so
> > that the guest kernel's log/dump message can be saved on the host
> > machine.  Users can access the log file directly on the host, or on the
> > guest at the next boot using pstore filesystem.  It currently deals with
> > kernel log (printk) buffer only, but we can extend it to have other
> > information (like ftrace dump) later.
> 
> Like the idea.

Thanks!

> 
> > 
> > It supports legacy PCI device using single order-2 page buffer.  As all
> 
> There should not be anything in there that limits this to pci, no?

Yep, there's no restriction AFAIK.  I just choose it to implement the poc
code quickly.

> 
> > operation of pstore is synchronous, it would be fine IMHO.  However I
> > don't know how to make write operation synchronous since it's called
> > with a spinlock held (from any context including NMI).
> > 
> > Cc: Paolo Bonzini <pbonzini@...hat.com>
> > Cc: Radim Kr??m???? <rkrcmar@...hat.com>
> > Cc: "Michael S. Tsirkin" <mst@...hat.com>
> > Cc: Anthony Liguori <aliguori@...zon.com>
> > Cc: Anton Vorontsov <anton@...msg.org>
> > Cc: Colin Cross <ccross@...roid.com>
> > Cc: Kees Cook <keescook@...omium.org>
> > Cc: Tony Luck <tony.luck@...el.com>
> > Cc: Steven Rostedt <rostedt@...dmis.org>
> > Cc: Ingo Molnar <mingo@...nel.org>
> > Cc: Minchan Kim <minchan@...nel.org>
> > Cc: kvm@...r.kernel.org
> > Cc: qemu-devel@...gnu.org
> > Cc: virtualization@...ts.linux-foundation.org
> > Signed-off-by: Namhyung Kim <namhyung@...nel.org>
> > ---
> >  drivers/virtio/Kconfig             |  10 ++
> >  drivers/virtio/Makefile            |   1 +
> >  drivers/virtio/virtio_pstore.c     | 317 +++++++++++++++++++++++++++++++++++++
> >  include/uapi/linux/Kbuild          |   1 +
> >  include/uapi/linux/virtio_ids.h    |   1 +
> >  include/uapi/linux/virtio_pstore.h |  53 +++++++
> >  6 files changed, 383 insertions(+)
> >  create mode 100644 drivers/virtio/virtio_pstore.c
> >  create mode 100644 include/uapi/linux/virtio_pstore.h
> > 
> 
> (...)
> 
> > diff --git a/drivers/virtio/virtio_pstore.c b/drivers/virtio/virtio_pstore.c
> > new file mode 100644
> > index 000000000000..6fe62c0f1508
> > --- /dev/null
> > +++ b/drivers/virtio/virtio_pstore.c
> > @@ -0,0 +1,317 @@
> > +#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
> > +
> > +#include <linux/kernel.h>
> > +#include <linux/module.h>
> > +#include <linux/pstore.h>
> > +#include <linux/virtio.h>
> > +#include <linux/virtio_config.h>
> > +#include <uapi/linux/virtio_ids.h>
> > +#include <uapi/linux/virtio_pstore.h>
> > +
> > +#define VIRT_PSTORE_ORDER    2
> > +#define VIRT_PSTORE_BUFSIZE  (4096 << VIRT_PSTORE_ORDER)
> 
> It may make sense to make the size of the buffer configurable through
> the config space.

Right.  I'm considering it too, but it needs a buffer larger than
kmsg_bytes (= 10K) to work properly in the current implementation.  As
this version is just to verify the idea is sane and useful, I used a
fixed size buffer.  Will change in the next version.

> 
> (...)
> 
> > diff --git a/include/uapi/linux/virtio_ids.h b/include/uapi/linux/virtio_ids.h
> > index 77925f587b15..cba63225d85a 100644
> > --- a/include/uapi/linux/virtio_ids.h
> > +++ b/include/uapi/linux/virtio_ids.h
> > @@ -41,5 +41,6 @@
> >  #define VIRTIO_ID_CAIF	       12 /* Virtio caif */
> >  #define VIRTIO_ID_GPU          16 /* virtio GPU */
> >  #define VIRTIO_ID_INPUT        18 /* virtio input */
> > +#define VIRTIO_ID_PSTORE       19 /* virtio pstore */
> 
> This id is already used by one of the new device types queued but not
> yet in the standard. IIRC, 22 is the next free one.

Ok, will update.

> 
> Speaking of the standard: I think it makes sense to at least reserve a
> device id for pstore, as the idea is sound. Maybe prepare a patch to
> the standard as well if you have time?

I'd love to.  As I mentioned earlier, I don't have enough knowledge in
this area.  Could you please provide some links about how can I do that?

Thanks,
Namhyung