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


From: Igor Mammedov
Subject: Re: [Qemu-devel] RFC: guest-side retrieval of fw_cfg file
Date: Thu, 16 Jul 2015 08:57:36 +0200

On Wed, 15 Jul 2015 19:24:33 +0300
"Michael S. Tsirkin" <address@hidden> 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" <address@hidden> 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. But if ACPI will start accessing fw_cfg from
other methods it will race for sure.




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