Re: [Xen-devel] [PATCH v5] x86/mm: Suppresses vm_events caused by page-walks

[Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>]

On 04/28/2018 12:30 AM, Tamas K Lengyel wrote:
> On Mon, Apr 23, 2018 at 2:00 AM, Alexandru Isaila
> <aisaila@xxxxxxxxxxxxxxx> wrote:
> > This patch is adding a way to enable/disable inguest pagefault
> > events. It introduces the xc_monitor_inguest_pagefault function
> > and adds the inguest_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_mem_access_check() we emulate so no event will get sent.
> This looks good to me, but is the emulator able to handle all
> instructions that may trigger it here?
That's a very good question. We think not, but we now have the 
UNIMPLEMENTED emulator event. The thought here is that the emulator 
would be able to handle most cases, and then the ones it can't handle we 
can handle with altp2m.
Of course, it's not ideal - we'd rather have a mechanism that's 
consistently foolproof, but I believe that Jan's objection is correct: 
we can't really be sure that the first time we get into access_check() 
with a specific [RIP:GLA] pair we need to set the A bit and the second 
time the D bit (interrupts may trip this logic up). Furthermore, with 
SVM the GLA is not available for page faults (although that's fixable by 
comparing GPAs).
If there's a better way to do this optimization that we haven't seen 
we'd be happy to switch the implementation. Suggestions are welcome.

Thanks,
Razvan

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