Re: [Xen-devel] QEMU e820 Reservation (FW_CFG_E820_TABLE and fw_cfg etc/e820)

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

On Wed, 2015-03-04 at 10:50 +0000, Stefano Stabellini wrote:
> On Wed, 4 Mar 2015, Gordan Bobic wrote:
> > On 2015-03-04 10:33, Stefano Stabellini wrote:
> > > On Wed, 4 Mar 2015, Gordan Bobic wrote:
> > > > Stefano,
> > > > 
> > > > Many thanks for responding to this. Resplies inline below.
> > > > 
> > > > On 2015-03-04 10:11, Stefano Stabellini wrote:
> > > > > On Tue, 3 Mar 2015, Gordan Bobic wrote:
> > > > > > Hi,
> > > > > >
> > > > > > I've been looking into custom e820 maps for domUs again, and
> > > > > > found that functionality to provide QEMU with hints regarding
> > > > > > e820 mapping has been upstream since some time in
> > > > > > 2010 (FW_CFG_E820_TABLE) with more finely grained control
> > > > > > (usable rather than just reserved entries) upstream since
> > > > > > 2013 (fw_cfg etc/e820).
> > > > > >
> > > > > > The respective patches are here:
> > > > > > http://lists.gnu.org/archive/html/qemu-devel/2010-02/msg00996.html
> > > > > > http://lists.gnu.org/archive/html/qemu-devel/2013-11/msg00593.html
> > > > > >
> > > > > > What I have not been able to find is any documentation at
> > > > > > all on how this e820 data can be given to QEMU when starting
> > > > > > a domain. I can see from the structs in the patches how the
> > > > > > data is packed for the relevant code to consume, but I cannot
> > > > > > figure out what is the delivery vector for this data. How
> > > > > > can I get QEMU to ingest the hints about any additional
> > > > > > reserved e820 blocks?
> > > > > >
> > > > > > For context, I need this to work around IOMMU implementation
> > > > > > bugs and mark areas of address space as reserved so that
> > > > > > the guest doesn't trample over the host's PCI I/O ranges
> > > > > > (which IOMMU should intercept, but being buggy, it doesn't).
> > > > > >
> > > > > > Many thanks in advance.
> > > > >
> > > > > Hello Gordan,
> > > > >
> > > > > FW_CFG_E820_TABLE is a special interface between SeaBios and QEMU but 
> > > > > is
> > > > > not used on Xen. I guess it could be made to work on Xen, but I am
> > > > > pretty sure it doesn't at the moment.
> > > > 
> > > > So this cannot be used to side-load an additional list of
> > > > e820 reserved memory blocks at domU startup time?
> > > 
> > > Nope.
> > > Enabling the usage of FW_CFG_E820_TABLE on Xen is conceivable and once
> > > done you would be able to use it, but today it would not work.
> > > 
> > > 
> > > > > I think you would probably want to look at hvmloader instead:
> > > > > tools/firmware/hvmloader/e820.c.
> > > > 
> > > > Yes, this is what I was looking at last time. I was just
> > > > hoping that either of the above mentioned patches could
> > > > be used to adjust the e820 map in a "soft" way rather
> > > > hard-coding any changes into hvmloader/e820.c The latter
> > > > is what I did last time, but it is extremely ugly and
> > > > non-generic.
> > > 
> > > I see.
> > > 
> > > 
> > > > And given the two interfaces I mentioned above it seems
> > > > really wrong to be implementing a whole new method for
> > > > manually loading an explicit e820 map. Is that not what
> > > > the etc/e820 interface is already supposed to do?
> > > 
> > > I don't follow you here: what is the etc/e820 interface?
> > 
> > See the 2nd patch I mentioned above, which supposedly
> > adds "etc/e820 fw_cfg file" (whatever that means).
> 
> Ah I see. Yes, this looks pretty much like what you need. We could
> expose something similar from xl/libxl and make use of this QEMU
> functionality on Xen. We would also need to coordinate with hvmloader
> but it should be doable.

I think going via qemu for this will turn out to be rather complex,
without a wholesale reworking of how (and by whom) the guest address map
is laid out.

Ian.


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