Re: [PATCH] kvm: device-assignment: Add PCI option ROM support

["Yang, Sheng" <sheng.yang@intel.com>]

On Friday 19 June 2009 00:28:41 Alex Williamson wrote:
> On Thu, 2009-06-18 at 13:30 +0800, Yang, Sheng wrote:
> > On Tuesday 16 June 2009 00:29:17 Alex Williamson wrote:
> > > The PCI code already knows about option ROMs, so we just need to
> > > mmap some space for it, load it with a copy of the contents, and
> > > complete the plubming to the generic code.
> > >
> > > With this a Linux guest can get access to the ROM contents via
> > > /sys/bus/pci/devices/<dev>/rom.  This might also enable the BIOS
> > > to execute ROMs by loading them dynamically from the device
> > > rather than hoping they all fit into RAM.
> >
> > The patch looks fine. One question: if guest write to the ROM, I think
> > the guest would be killed for QEmu would receive a SIGSEGV? I am not sure
> > if it's too severe...
>
> Hi Sheng,
>
> Good thought.  I tested this with a simple program that mmaps the ROM
> address from /dev/mem and tries to write to it (using setpci to enable
> the ROM).  The results are a little surprising.  On the host, writing to
> the ROM causes an NMI and the system dies.  On the KVM guest, the write
> is happily discarded, neither segfaulting from the mprotect nor
> affecting the contents of the ROM.  So it seems that something above my
> mprotect is discarding the write, and if we did hit it, a qemu segfault
> isn't that far from what happens on bare metal.
>
Oh, that's good. :)

> The one oddity I noticed is that even when the enable bit is clear, the
> guest can read the ROM.  I don't know that this is actually illegal, vs
> returning zeros or ones though.  It seems like maybe the generic PCI
> code isn't tracking the enable bit.  I think that's an independent
> problem from this patch though.  Thanks,

That should be fine. I've taken a look at code, seems Linux kernel set 
enable_bit when someone begin to read rom, and copy rom to buffer, then unmap 
the rom. So the rom can be read when enable bit clear.

-- 
regards
Yang, Sheng

>
> Alex