On 27.11.2019 18:11, Paul Durrant wrote:
This patch introduces a new iommu_op to facilitate a per-implementation
quarantine set up, and then further code for x86 implementations
(amd and vtd) to set up a read-only scratch page to serve as the source
for DMA reads whilst a device is assigned to dom_io. DMA writes will
continue to fault as before.
The reason for doing this is that some hardware may continue to re-try
DMA (despite FLR) in the event of an error, or even BME being cleared, and
will fail to deal with DMA read faults gracefully. Having a scratch page
mapped will allow pending DMA reads to complete and thus such buggy
hardware will eventually be quiesced.
NOTE: These modifications are restricted to x86 implementations only as
the buggy h/w I am aware of is only used with Xen in an x86
environment. ARM may require similar code but, since I am not
aware of the need, this patch does not modify any ARM implementation.
Signed-off-by: Paul Durrant <pdurrant@xxxxxxxxxx>
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
There is still the open question of whether use of a scratch page ought
to be gated on something, either are run-time or compile-time.
I have no clear opinion either way here. The workaround seems low
overhead enough that there may not be a need to have an admin (or
build time) control for this.
As to 4.13: The quarantining as a whole is pretty fresh. While it
has been backported to security maintained trees, I'd still consider
it a new feature in 4.13, and hence this workaround at least eligible
for consideration.
