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

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

On Thu, Jul 16, 2015 at 11:50:38AM +0200, Igor Mammedov wrote:
> On Thu, 16 Jul 2015 10:30:41 +0300
> "Michael S. Tsirkin" <mst@redhat.com> wrote:
> 
> > 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?
> I've thought that it's pretty static and doesn't change.

vm gen id? No, its dynamic.

> > 
> > > 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.
> Yep, maybe we shouldn't.
> 
> The last vmgenid series just maps UUID region
> at fixed location (which is sufficient to build ACPI
> tables at machine done time) so there isn't real need
> to export address via fw_cfg.
> 

Other versions just write into guest memory from QEMU,
that will work fine too.

-- 
MST