Re: [Qemu-devel] QEMU etc/e820 and fw_cfg

[Gordan Bobic]

On 2015-03-05 10:42, Gerd Hoffmann wrote:
> Hi,
>
> > >> Thank you for responding. The situation I have is that my PCIe
> > >> bridges are buggy and they seem to bypass the upstream PCIe hub
> > >> IOMMU. The problem with this is that when the guest accesses
> > >> RAM within it's emulated address space that overlaps with
> > >> PCI I/O memory ranges in the host's address space, what should
> > >> have ended up in RAM in the guest ends up trampling over the
> > >> IOMEM on the host.
> > >
> > > The iommu isn't involved here at all.  When the pci devices are
> > > accessing host ram via busmaster dma, *this* goes through the iommu.
> > > And unless you are trying to use pci device assignment the iommu should
> > > not matter at all.
> >
> > I am using PCI device assignment. I'm passing a PCI devices to the
> > guest VM.
>
> Oh.  I didn't expect someone trying to use device assign with a
> known-broken iommu.  /me looks surprised.

Since all I have is lemons I'm trying to make lemonade. :)

> > I'm pretty sure I am dealing with a hardware bug here. I have
> > a workaround that I know works (mark the host's IOMEM areas
> > as reserved) - I just need a way to get QEMU to adjust the
> > exposed e820 map accordingly.
>
> Add "e820_add_entry(start, size, E820_RESERVED)" calls in qemu.

Could you please point me at the correct file/function to add
the relevant block into?

I would probably look to do add these based on a config file
in /etc/qemu/. Happy to forward a patch for inclusion if I
manage to make it work.

> Also make sure the firmware doesn't use those ranges, which may need
> firmware patching.  At least seabios should happily add those
> reservations to the e820 map, but will not look at them otherwise, so
> you could end up with pci bars being mapped within the reserved 
> regions.

Are you saying that seabios will find reserved areas in the e820
map and despite that map a BAR into a reserved block? That's pretty
broken...

> The linux kernel might fix it up at boot though.

If you mean inside the VM, Linux-on-Linux isn't my intended use case,
though.

> Not fully sure how OVMF behaves here.

Thanks for your input. I'll find an appropriate place to ask
about OVMF once I have the QEMU patched appropriately.

Gordan