Re: [Qemu-devel] RFC: guest-side retrieval of fw_cfg file

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Jul 16, 2015 at 08:57:36AM +0200, Igor Mammedov wrote:
> On Wed, 15 Jul 2015 19:24:33 +0300
> "Michael S. Tsirkin" <mst@redhat.com> wrote:
> 
> > On Wed, Jul 15, 2015 at 06:01:42PM +0200, Igor Mammedov wrote:
> > > On Wed, 15 Jul 2015 17:08:27 +0300
> > > "Michael S. Tsirkin" <mst@redhat.com> wrote:
> > > 
> > > > On Mon, Jul 13, 2015 at 04:09:37PM -0400, Gabriel L. Somlo wrote:
> > > > > Hi,
> > > > > 
> > > > > A while ago I was pondering on the options available for retrieving
> > > > > a fw_cfg blob from the guest-side (now that we can insert fw_cfg
> > > > > files on the host-side command line, see commit 81b2b8106).
> > > > > 
> > > > > So over the last couple of weekends I cooked up the sysfs kernel
> > > > > module below, which lists all fw_cfg files
> > > > > under /sys/firmware/fw_cfg/<filename>.
> > > > 
> > > > One concern here is that there will be a conflict here if fw cfg
> > > > is used by ACPI. I don't know whether that last is a good idea
> > > > though, so maybe not a real concern. I think Igor
> > > > wanted to make it so.
> > > 
> > > I don't see any conflict here so far, it's just guest side module that
> > > accesses fw_cfg.
> > 
> > If there's ACPI code that accesses fw_cfg in response to an interrupt,
> > it'll race with fw cfg access by guest OS. On linux we might be able to
> > block ACPI preventing it from running. We probably won't be able to do
> > it on windows.
> wrt vmgenid series we were talking about possibility to access fw_cfg
> from ACPI device.init so it's unlikely that it will ever collide with
> much later sysfs accesses.

Don't we need to get updates when it changes too?

> But if ACPI will start accessing fw_cfg from
> other methods it will race for sure.

Maybe we shouldn't poke at fw cfg in ACPI then.

-- 
MST