Re: [Xen-devel] [PATCH 1/2] VMX: fix VMCS race on context-switch paths

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 15.02.17 at 12:55, <JBeulich@xxxxxxxx> wrote:
>>>> On 15.02.17 at 12:48, <sergey.dyasli@xxxxxxxxxx> wrote:
>> On Wed, 2017-02-15 at 04:39 -0700, Jan Beulich wrote:
>>> > > > On 15.02.17 at 11:27, <sergey.dyasli@xxxxxxxxxx> wrote:
>>> > (XEN) [ 1408.075638] Xen call trace:
>>> > (XEN) [ 1408.079322]    [<ffff82d0801ea2a2>] vmx_vmcs_reload+0x32/0x50
>>> > (XEN) [ 1408.086303]    [<ffff82d08016c58d>] context_switch+0x85d/0xeb0
>>> > (XEN) [ 1408.093380]    [<ffff82d08012fb8e>] 
>>> > schedule.c#schedule+0x46e/0x7d0
>>> > (XEN) [ 1408.100942]    [<ffff82d080164305>] reprogram_timer+0x75/0xe0
>>> > (XEN) [ 1408.107925]    [<ffff82d080136400>] 
> timer.c#timer_softirq_action+0x90/0x210
>>> > (XEN) [ 1408.116263]    [<ffff82d08013311c>] 
>>> > softirq.c#__do_softirq+0x5c/0x90
>>> > (XEN) [ 1408.123921]    [<ffff82d080167d35>] domain.c#idle_loop+0x25/0x60
>>> 
>>> Taking your later reply into account - were you able to figure out
>>> what other party held onto the VMCS being waited for here?
>> 
>> Unfortunately, no. It was unclear from debug logs. But judging from
>> the following vmx_do_resume() code:
>> 
>>     if ( v->arch.hvm_vmx.active_cpu == smp_processor_id() )
>>     {
>>         if ( v->arch.hvm_vmx.vmcs_pa != this_cpu(current_vmcs) )
>>             vmx_load_vmcs(v);
>>     }
>> 
>> If both of the above conditions are true then vmx_vmcs_reload() will
>> probably hang.
> 
> I don't follow (reload should run before this, not after), but I must
> be missing something more general anyway, as I'm seeing the code
> above being needed despite the reload additions.

I think I've understood part of it over lunch: Surprisingly enough
vmx_ctxt_switch_to() doesn't re-establish the VMCS, so it needs
to be done here. Which I think means we don't need the new
hook at all, as that way the state is no different between going
through ->to() or bypassing it.

What I continue to not understand is why vmcs_pa would ever
not match current_vmcs when active_cpu is smp_processor_id().
So far I thought both are always updated together. Looking
further ...

Jan


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