[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [Xen-devel] [PATCH v2 1/5] hvmloader: Correct bug in low mmio region accounting

On 06/18/2013 05:46 PM, George Dunlap wrote:
When deciding whether to map a device in low MMIO space (<4GiB),
hvmloader compares it with "mmio_left", which is set to the size of
the low MMIO range (pci_mem_end - pci_mem_start).  However, even if it
does map a device in high MMIO space, it still removes the size of its
BAR from mmio_left.

This patch first changes the name of this variable to "low_mmio_left"
to distinguish it from generic MMIO, and corrects the logic to only
subtract the size of the BAR for devices maped in the low MMIO region.

Also make low_mmio_left unsigned, and don't allow it to go negative.
Since its main use is to be compared to a 64-bit unsigned int, this
may have undefined (and in practice almost certainly incorrect)
results.  Not subtracting is OK because if there's not enough room, it
won't actually be mapped.

Signed-off-by: George Dunlap <george.dunlap@xxxxxxxxxxxxx>

This patch series is a lot longer and more intrusive than I was hoping it would be. :-(

However, it still seems to me that this will be a smaller impact on people than the alternatives.

If the code is done properly, then none of these changes should have any effect on people who are not passing through devices with large BARs: the vast majority of our users, who are simply booting plain VMs in HVM mode, should see zero effect.

Similarly, the vast majority of qemu-traditional users should also see no effect; the 64-bit code should only be invoked when the MMIO hole cannot be resized large enough, and by default, qemu-traditional will be able to resize the hole up to 2GiB -- far larger than almost any guest should need.

qemu-xen users will be affected, but they would have been affected anyway. This changes a certain "will 100% crash at some point" to "will probably work most of the time, with a small unforeseen chance of something crashing randomly". I think that's still a step forward.


Xen-devel mailing list



Lists.xenproject.org is hosted with RackSpace, monitoring our
servers 24x7x365 and backed by RackSpace's Fanatical Support®.