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

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Message-ID: <20161115163342-mutt-send-email-mst@kernel.org>
Date:   Tue, 15 Nov 2016 16:35:06 +0200
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Namhyung Kim <namhyung@...nel.org>
Cc:     virtio-dev@...ts.oasis-open.org, kvm@...r.kernel.org,
        qemu-devel@...gnu.org, virtualization@...ts.linux-foundation.org,
        LKML <linux-kernel@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...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>
Subject: Re: [PATCH 1/3] virtio: Basic implementation of virtio pstore driver

On Tue, Nov 15, 2016 at 02:50:11PM +0900, Namhyung Kim wrote:
> On Tue, Nov 15, 2016 at 07:06:28AM +0200, Michael S. Tsirkin wrote:
> > On Tue, Nov 15, 2016 at 01:50:21PM +0900, Namhyung Kim wrote:
> > > On Thu, Nov 10, 2016 at 06:39:55PM +0200, Michael S. Tsirkin wrote:
> > > [SNIP]
> > > > > +struct virtio_pstore_fileinfo {
> > > > > +	__virtio64		id;
> > > > > +	__virtio32		count;
> > > > > +	__virtio16		type;
> > > > > +	__virtio16		unused;
> > > > > +	__virtio32		flags;
> > > > > +	__virtio32		len;
> > > > > +	__virtio64		time_sec;
> > > > > +	__virtio32		time_nsec;
> > > > > +	__virtio32		reserved;
> > > > > +};
> > > > > +
> > > > > +struct virtio_pstore_config {
> > > > > +	__virtio32		bufsize;
> > > > > +};
> > > > > +
> > > > 
> > > > What exactly does each field mean? I'm especially
> > > > interested in time fields - maintaining a consistent
> > > > time between host and guest is not a simple problem.
> > > 
> > > These are required by pstore and will be used to create corresponding
> > > files in the pstore filesystem.  The time fields are for mtime and
> > > ctime and, I think, it's just a hint for user and doesn't require
> > > strict consistency.
> > 
> > Pls add documentation. I would just drop hints for now.
> 
> Well, I'll add docmentation.  But I think just dropping might not good
> since they all have host time and it's helpful to know their relative
> difference in guest.
> 
> Thanks,
> Namhyung

If it's part of host/guest ABI it needs to be better defined.
"It's just a hint does not need to be exact" is too vague,
we need to specify what kind of change will or will not
break guests.

-- 
MST