[Xen-devel] Locking on vm-event operations (monitor)

[Corneliu ZUZU <czuzu@xxxxxxxxxxxxxxx>]

Hi,

I've been inspecting vm-event code parts to try and understand when and 
why domain pausing/locking is done. If I understood correctly, domain 
pausing is done solely to force all the vCPUs of that domain to see a 
configuration update and act upon it (e.g. in the case of a 
XEN_DOMCTL_MONITOR_EVENT_WRITE_CTRLREG that enables CR3 monitoring, 
domain pausing/unpausing ensures immediate enabling of CR3 load-exiting 
for all vCPUs), not to synchronize concurrent operations (lock-behavior).
As for locking, I see that for example vm_event_init_domain(), 
vm_event_cleanup_domain() and monitor_domctl() are all protected by the 
domctl-lock, but I don't think that's enough.
Here are a few code-paths that led me to believe that:
* do_hvm_op()->monitor_guest_request() reads d.monitor.guest_request_* 
resources, but it doesn't seem to take the domctl lock, so it seems 
possible for that to happen _while_ those resources are 
initialized/cleaned-up
* monitor_guest_request() also calls 
monitor_traps()->...->vm_event_wait_slot()->...->vm_event_grab_slot() 
which attempts a vm_event_ring_lock(ved), which could also be called 
_while_ that's initialized (vm_event_enable()) or cleaned-up 
(vm_event_disable())
* hvm_monitor_cr() - e.g. on the code-path 
vmx_vmexit_handler(EXIT_REASON_CR_ACCESS)->vmx_cr_access(VMX_CONTROL_REG_ACCESS_TYPE_MOV_TO_CR)->hvm_mov_to_cr()->hvm_set_cr0()->hvm_monitor_crX() 
there doesn't seem to be taken into account the possibility of a 
concurrent monitor_init_domain()/monitor_cleanup_domain()
Am I missing something with these conclusions?
As a resolution for this, I've been thinking of adding a 'subsys_lock' 
field in the vm_event_domain structure, either spinlock or rw-lock, 
which would be initialised/uninitialised when d.vm_event is 
allocated/freed (domain_create()/complete_domain_destroy()).
Let me know what you think,
Corneliu.

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