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

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Mon, Dec 24, 2018 at 09:04:35AM +0100, Roger Pau Monné wrote:
> 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.)
> 
> Thanks!
> 
> I have the results of an ad-hoc osstest flight with the change:
> 
> http://logs.test-lab.xenproject.org/osstest/logs/131498/
> 
> It doesn't looks like there are any regressions.

Given the flight is green and George is likely to be on vacation ATM. I
will commit this patch at some point.

Wei.

> 
> Roger.

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