Re: [Qemu-devel] [PATCH 1/3] virtio: Basic implementation of virtio pstore driver

[Namhyung Kim]

On Tue, Jul 19, 2016 at 10:43 PM, Namhyung Kim <address@hidden> wrote:
> Hi Kees,
>
> On Mon, Jul 18, 2016 at 10:50:06AM -0700, Kees Cook wrote:
>> On Sun, Jul 17, 2016 at 10:50 PM, Namhyung Kim <address@hidden> wrote:
>> > Hello,
>> >
>> > On Sun, Jul 17, 2016 at 10:12:26PM -0700, Kees Cook wrote:
>> >> On Sun, Jul 17, 2016 at 9:37 PM, Namhyung Kim <address@hidden> wrote:
>> > [SNIP]
>> >> > +static u16 to_virtio_type(struct virtio_pstore *vps, enum 
>> >> > pstore_type_id type)
>> >> > +{
>> >> > +       u16 ret;
>> >> > +
>> >> > +       switch (type) {
>> >> > +       case PSTORE_TYPE_DMESG:
>> >> > +               ret = cpu_to_virtio16(vps->vdev, 
>> >> > VIRTIO_PSTORE_TYPE_DMESG);
>> >> > +               break;
>> >> > +       default:
>> >> > +               ret = cpu_to_virtio16(vps->vdev, 
>> >> > VIRTIO_PSTORE_TYPE_UNKNOWN);
>> >> > +               break;
>> >> > +       }
>> >>
>> >> I would love to see this support PSTORE_TYPE_CONSOLE too. It should be
>> >> relatively easy to add: I think it'd just be another virtio command?
>> >
>> > Do you want to append the data to the host file as guest does
>> > printk()?  I think it needs some kind of buffer management, but it's
>> > not hard to add IMHO.
>>
>> Well, with most pstore backends, the buffer size is limited, so it
>> tends to be a circular buffer of some sort. I think whatever you
>> choose to do is fine (I saw the various mentions of resource limits in
>> the qemu part of this thread), as long as the last N bytes of console
>> can be seen on the host side, where N is some portion of the memory
>> set aside for the log. (I don't mind the idea of an unlimited console
>> log either, but I suspect that will not be accepted on the qemu
>> side...)
>
> I think it needs two kinds of buffer management.
>
> The first one is the psinfo->buf (or something similar).  IIUC the
> PSTORE_TYPE_CONSOLE is different than PSTORE_TYPE_DMESG as it is
> emitted every time printk() sends messages to console.  So I think the
> it should remain in async mode due to performance reason.  To do that,
> the message should be copied to psinfo->buf and then sent via virtio.
> Then it needs to keep track of the available buffer position IMHO.

Looking at the code, I found myself confused with the PSTORE_TYPE_FTRACE
and PSTORE_TYPE_CONSOLE.  It seems that handling of
PSTORE_TYPE_CONSOLE is basically same as PSTORE_TYPE_DMESG..

Thanks,
Namhyung