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

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Date:	Mon, 18 Jul 2016 09:54:39 +0200
From:	Cornelia Huck <cornelia.huck@...ibm.com>
To:	Namhyung Kim <namhyung@...nel.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Radim Krčmář <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

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.

> 
> 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?

> 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 Krčmář <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.

(...)

> 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.

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?

>  
>  #endif /* _LINUX_VIRTIO_IDS_H */