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Re: [Xen-devel] [PATCH v2 3/3] x86/mm-locks: apply a bias to lock levels for control domain

On Fri, Dec 21, 2018 at 06:37:32PM +0000, George Dunlap wrote:
> > On Dec 21, 2018, at 9:41 AM, Roger Pau Monne <roger.pau@xxxxxxxxxx> wrote:
> > 
> > paging_log_dirty_op function takes mm locks from a subject domain and
> > then attempts to perform copy to operations against the caller domain
> > in order to copy the result of the hypercall into the caller provided
> > buffer.
> > 
> > This works fine when the caller is a non-paging domain, but triggers a
> > lock order panic when the caller is a paging domain due to the fact
> > that at the point where the copy to operation is performed the subject
> > domain paging lock is locked, and the copy operation requires
> > locking the caller p2m lock which has a lower level.
> > 
> > Fix this limitation by adding a bias to the level of control domain mm
> > locks, so that the lower control domain mm lock always has a level
> > greater than the higher unprivileged domain lock level. This allows
> > locking the subject domain mm locks and then locking the control
> > domain mm locks, while keeping the same lock ordering and the changes
> > mostly confined to mm-locks.h.
> > 
> > Note that so far only this flow (locking a subject domain locks and
> > then the control domain ones) has been identified, but not all
> > possible code paths have been inspected. Hence this solution attempts
> > to be a non-intrusive fix for the problem at hand, without discarding
> > further changes in the future if other valid code paths are found that
> > require more complex lock level ordering.
> > 
> > Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> Reviewed-by: George Dunlap <george.dunlap@xxxxxxxxxx>
> …but given the nature of the change, I’d like to see it pass at least one 
> ad-hoc osstest run before it gets checked in.  (That probably means you’ll 
> have to chase someone else to do that.)


I have the results of an ad-hoc osstest flight with the change:


It doesn't looks like there are any regressions.


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