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

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

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.

I think you would probably want to look at hvmloader instead:
tools/firmware/hvmloader/e820.c.

Cheers,

Stefano

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