[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 

Does that sound reasonable?

Not only does that sound reasonable, I personally prefer this approach. I'll apply this patch and give it a spin.


Xen-devel mailing list



Lists.xenproject.org is hosted with RackSpace, monitoring our
servers 24x7x365 and backed by RackSpace's Fanatical Support®.