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Re: [Xen-devel] XSA-60 - how to get back to a sane state



On 12/04/2013 12:16 PM, Jan Beulich wrote:
On 04.12.13 at 13:04, "Liu, Jinsong" <jinsong.liu@xxxxxxxxx> wrote:
Jan Beulich wrote:
On 03.12.13 at 15:30, "Liu, Jinsong" <jinsong.liu@xxxxxxxxx> wrote:
Jan Beulich wrote:
On 03.12.13 at 04:06, "Liu, Jinsong" <jinsong.liu@xxxxxxxxx>
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

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