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

>>> On 16.02.17 at 13:27, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 16/02/17 11:15, Jan Beulich wrote:
>> When __context_switch() is being bypassed during original context
>> switch handling, the vCPU "owning" the VMCS partially loses control of
>> it: It will appear non-running to remote CPUs, and hence their attempt
>> to pause the owning vCPU will have no effect on it (as it already
>> looks to be paused). At the same time the "owning" CPU will re-enable
>> interrupts eventually (the lastest when entering the idle loop) and
>> hence becomes subject to IPIs from other CPUs requesting access to the
>> VMCS. As a result, when __context_switch() finally gets run, the CPU
>> may no longer have the VMCS loaded, and hence any accesses to it would
>> fail. Hence we may need to re-load the VMCS in vmx_ctxt_switch_from().
>> Similarly, when __context_switch() is being bypassed also on the second
>> (switch-in) path, VMCS ownership may have been lost and hence needs
>> re-establishing. Since there's no existing hook to put this in, add a
>> new one.
>> Reported-by: Kevin Mayer <Kevin.Mayer@xxxxxxxx>
>> Reported-by: Anshul Makkar <anshul.makkar@xxxxxxxxxx>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> Although I would certainly prefer if we can get another round of testing
> on this series for confidence.

Sure, I'd certainly like to stick a Tested-by on it. Plus VMX maintainer
feedback will need waiting for anyway.


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