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SubjectRe: [PATCH] KVM: VMX: Enable Notify VM exit
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On 5/17/2021 3:20 PM, Xiaoyao Li wrote:
> Hi Sean, Andy and Paolo,

+ real Sean

> On 11/3/2020 2:33 AM, Sean Christopherson wrote:
>> On Mon, Nov 02, 2020 at 10:01:16AM -0800, Andy Lutomirski wrote:
>>> On Mon, Nov 2, 2020 at 9:31 AM Sean Christopherson
>>> <sean.j.christopherson@intel.com> wrote:
>>>>
>>>> Tao, this patch should probably be tagged RFC, at least until we can
>>>> experiment
>>>> with the threshold on real silicon.  KVM and kernel behavior may
>>>> depend on the
>>>> accuracy of detecting actual attacks, e.g. if we can set a threshold
>>>> that has
>>>> zero false negatives and near-zero false postives, then it probably
>>>> makes sense
>>>> to be more assertive in how such VM-Exits are reported and logged.
>>>
>>> If you can actually find a threshold that reliably mitigates the bug
>>> and does not allow a guest to cause undesirably large latency in the
>>> host, then fine.  1/10 if a tick is way too long, I think.
>>
>> Yes, this was my internal review feedback as well.  Either that got
>> lost along
>> the way or I wasn't clear enough in stating what should be used as a
>> placeholder
>> until we have silicon in hand.
>>
>
> We have tested on real silicon and found it can work even with threshold
> being set to 0.
>
> It has an internal threshold, which is added to vmcs.notify_window as
> the final effective threshold. The internal threshold is big enough to
> cover normal instructions. For those long latency instructions like
> WBINVD, the processor knows they cannot cause no interrupt window
> attack. So no Notify VM exit will happen on them.
>
> Initially, our hardware architect wants to set the notify window to
> scheduler tick to not break kernel scheduling. But you folks want a
> smaller one. So are you OK to set the window to 0?
>
>

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