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 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?

Thanks,
Tamas

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