Re: XSA-60 - how to get back to a sane state

[George Dunlap <george.dunlap@eu.citrix.com>]

On 12/04/2013 12:16 PM, Jan Beulich wrote:
>>>> On 04.12.13 at 13:04, "Liu, Jinsong" <jinsong.liu@intel.com> wrote:
>> Jan Beulich wrote:
>>>>>> On 03.12.13 at 15:30, "Liu, Jinsong" <jinsong.liu@intel.com> wrote:
>>>> Jan Beulich wrote:
>>>>>>>> On 03.12.13 at 04:06, "Liu, Jinsong" <jinsong.liu@intel.com>
>>>>>>>> wrote:
>>>>>> I also vote option 2, but only revert 86d60e85, keeping 62652c00
>>>>>> (wbinvd at vmx_ctxt_switch_to) since it's used to avoid being
>>>>>> polluted when vcpu migrate to another cpu.
>>>>> Please explain this in more detail. Both Andrew and I are concerned
>>>>> about this extra, but pretty pointless (without being done so too in
>>>>> other cases) wbinvd(). In particular you'd have to explain what its
>>>>> counterpart was in the code prior to your four patch XSA-60 series.
>>>> The wbinvd at vmx_ctxt_switch_to is for case like
>>>> 1. vcpu runs at cpu A, flushing cache at vmx_handle_cd;
>>>> 2. then the vcpu may switch out and migrate to cpu B;
>>>> 3. historically cpu B may has cacheline polluted;
>>>> so when the vcpu is scheduled to cpu B, we need flush cache.
>>> But you didn't clarify whether/how this case was taken care of
>>> _before_ your XSA-60 patches.
>>>
>> I didn't understand your question. What do you mean by 'before my XSA-60
>> patches'?
> Before your 4 patch series was applied (e.g. consider plain
> 4.3.1) - how was the situation taken care of that your change
> to vmx_ctxt_switch_to() is intended to deal with?

It sounds like Jan is saying: We would only consider a patch that would 
fix regressions in functionality caused by the 4-patch XSA-60 series.  
Was there the possibility for cacheline pollution in the scenario you 
describe above before XSA-60 was fixed?  If not, then this is a 
regression and we might consider a patch to restore that functionality.  
If there was the possibility of the above scenario before the XSA-60 
series, then it's not a regression; and therefore probably not something 
we want to accept at this point.

Do I understand you properly, Jan?

  -George