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

[Gordan Bobic <gordan@xxxxxxxxxx>]

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?

> 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.

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?

Gordan

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