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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [PATCH for-4.12 V4] x86/altp2m: fix HVMOP_altp2m_set_domain_state race
> On Feb 12, 2019, at 11:42 AM, Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx> > wrote: > > HVMOP_altp2m_set_domain_state does not domain_pause(), presumably > on purpose (as it was originally supposed to cater to a in-guest > agent, and a domain pausing itself is not a good idea). Sorry to come in here on v4 and suggest changing everything, but I don’t really like the solution you have here. Not setting altp2m to ‘active’ until after the vcpus are set up makes sense; but passing this true/false value in seems ugly, and still seems a bit racy (i.e., what if p2m_active() is disabled between the check in HVMOP_altp2m_switch_p2m and the time we actually call altp2m_vcpu_update_p2m()?) I certainly don’t think domain_pause() should be our go-to solution for race avoidance, but in this case it really seems like switching the global p2m for every vcpu at once makes the most sense; and trying to safely change this on an unpaused domain is not only overly complicated, but probably not what we wanted anyway. p2m_altp2m_destroy_by_id() and p2m_switch_domain_altp2m_by_id() already use domain_pause_except_self(); so it seems like not using it for altp2m_set_domain_state was probably more of an oversight than an intentional decision. Using that here seems like a more robust solution. The one issue is that domain_pause_except_self() currently is actually a deadlock risk if two different vcpus start it at the same time. I think the attached patch (compile-tested only) should fix this issue; after this patch you should be able to use domain_pause_except_self() in altp2m_set_domain_state instead. Does that sound reasonable? -George Attachment:
0001-altp2m-Prevent-deadlocks-when-a-domain-performs-altp.patch _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxxxxxxxxx https://lists.xenproject.org/mailman/listinfo/xen-devel
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