Re: [Xen-devel] [PATCH v2 1/2] VMX: fix VMCS race on context-switch paths

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 09.11.17 at 10:54, <raistlin@xxxxxxxx> wrote:
> On Tue, 2017-11-07 at 14:24 +0000, Igor Druzhinin wrote:
>> Perhaps I should improve my diagram:
>> 
>> pCPU1: vCPUx of domain X -> migrate to pCPU2 -> switch to idle
>> context
>> -> RCU callbacks -> vcpu_destroy(vCPUy of domain Y) ->
>> vmx_vcpu_disable_pml() -> vmx_vmcs_clear() (VMCS is trashed at this
>> point on pCPU1)
>> 
>> pCPU2: context switch into vCPUx -> vCPUx.is_running = 1 -> TLB flush
>> from context switch to clean TLB on pCPU1
>> 
> Sorry, there must be something I'm missing (or misunderstanding).
> 
> What is this code that checks is_running and triggers the TLB flush?

I don't see where Igor said is_running is being checked around a
TLB flush. The TLB flush itself is what happens first thing in
context_switch() (and it's really using the TLB flush interface to
mainly effect the state flush, with the TLB flush being an implied
side effect; I've already got a series of further patches to make
this less implicit).

> But, more important, how come you are context switching to something
> that has is_running == 1 ? That should not be possible.

That's not what Igor's diagram says - it's indicating the fact that
is_running is being set to 1 in the process of context switching
into vCPUx.

Jan


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