On 2015/12/14 12:54, Alexander Duyck wrote:
On Sun, Dec 13, 2015 at 6:27 PM, Yang Zhang <yang.zhang.wz@xxxxxxxxx> wrote:
On 2015/12/14 5:28, Alexander Duyck wrote:
This patch set is meant to be the guest side code for a proof of concept
involving leaving pass-through devices in the guest during the warm-up
phase of guest live migration. In order to accomplish this I have added a
new function called dma_mark_dirty that will mark the pages associated
with
the DMA transaction as dirty in the case of either an unmap or a
sync_.*_for_cpu where the DMA direction is either DMA_FROM_DEVICE or
DMA_BIDIRECTIONAL. The pass-through device must still be removed before
the stop-and-copy phase, however allowing the device to be present should
significantly improve the performance of the guest during the warm-up
period.
This current implementation is very preliminary and there are number of
items still missing. Specifically in order to make this a more complete
solution we need to support:
1. Notifying hypervisor that drivers are dirtying DMA pages received
2. Bypassing page dirtying when it is not needed.
Shouldn't current log dirty mechanism already cover them?
The guest has no way of currently knowing that the hypervisor is doing
dirty page logging, and the log dirty mechanism currently has no way
of tracking device DMA accesses. This change is meant to bridge the
two so that the guest device driver will force the SWIOTLB DMA API to
mark pages written to by the device as dirty.
OK. This is what we called "dummy write mechanism". Actually, this is
just a workaround before iommu dirty bit ready. Eventually, we need to
change to use the hardware dirty bit. Besides, we may still lost the
data if dma happens during/just before stop and copy phase.
The two mechanisms referenced above would likely require coordination with
QEMU and as such are open to discussion. I haven't attempted to address
them as I am not sure there is a consensus as of yet. My personal
preference would be to add a vendor-specific configuration block to the
emulated pci-bridge interfaces created by QEMU that would allow us to
essentially extend shpc to support guest live migration with pass-through
devices.
The functionality in this patch set is currently disabled by default. To
enable it you can select "SWIOTLB page dirtying" from the "Processor type
and features" menu.
Only SWIOTLB is supported?
Yes. For right now this only supports SWIOTLB. The assumption here
is that SWIOTLB is in use for most cases where an IOMMU is not
present. If an IOMMU is present in a virtualized guest then most
likely the IOMMU might be able to provide a separate mechanism for
dirty page tracking.
- Alex
--
best regards
yang
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