Re: [Xen-devel] [PATCH v1] x86/mm: Supresses vm_events caused by page-walks

[Tamas K Lengyel <tamas@xxxxxxxxxxxxx>]

On Mon, Oct 30, 2017 at 11:01 AM, Razvan Cojocaru
<rcojocaru@xxxxxxxxxxxxxxx> wrote:
> On 10/30/2017 06:39 PM, Tamas K Lengyel wrote:
>> On Mon, Oct 30, 2017 at 10:24 AM, Razvan Cojocaru
>> <rcojocaru@xxxxxxxxxxxxxxx> wrote:
>>> On 30.10.2017 18:01, Tamas K Lengyel wrote:
>>>> On Mon, Oct 30, 2017 at 4:32 AM, Alexandru Isaila
>>>> <aisaila@xxxxxxxxxxxxxxx> wrote:
>>>>> This patch is adding a way to enable/disable nested pagefault
>>>>> events. It introduces the xc_monitor_nested_pagefault function
>>>>> and adds the nested_pagefault_disabled in the monitor structure.
>>>>> This is needed by the introspection so it will only get gla
>>>>> faults and not get spammed with other faults.
>>>>> In p2m_set_ad_bits the v->arch.sse_pg_dirty.eip and
>>>>> v->arch.sse_pg_dirty.gla are used to mark that this is the
>>>>> second time a fault occurs and the dirty bit is set.
>>>>
>>>> Could you describe under what conditions do you get these other faults?
>>>
>>> Hey Tamas, the whole story is at page 8 of this document:
>>>
>>> https://www.researchgate.net/publication/281835515_Proposed_Processor_Extensions_for_Significant_Speedup_of_Hypervisor_Memory_Introspection
>>
>> Hi Razvan,
>> thanks but I'm not sure that doc addresses my question. You
>> effectively filter out npfec_kind_in_gpt and npfec_kind_unknown in
>> this patch. The first, npfec_kind_in_gpt should only happen if you
>> have restricted access to the gpt with ept and the processor couldn't
>> walk the table. But if you don't want to get events of these types
>> then why not simply not restrict access the gpt to begin with? And as
>> for npfec_kind_unknown, I don't think that gets generated under any
>> situation. So hence my question, what is your setup that makes this
>> patch necessary?
>
> On the npfec_kind_unknown case, indeed, we were wondering when that
> might possibly occur when discussing this patch - it's probably reserved
> for the future?
>
> On why our introspection engine decides to restrict access to those
> specific pages, I am not intimate with its inner workings, and not sure
> how much could be disclosed here in any case. Is it not a worthwhile
> (and otherwise harmless) tool to be able to switch A/D bits-triggered
> EPT faults anyway, for introspection purposes?

It changes the default behavior of mem_access events so I just wanted
to get some background on when that is really required. Technically
there is no reason why we couldn't do that filtering in Xen. I think
it might be better to flip the filter the other way though so the
default behavior remains as is (ie. change the option to enable
filtering instead of enabling monitoring).

Tamas

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