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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 28.04.18 at 08:13, <rcojocaru@xxxxxxxxxxxxxxx> wrote:
> 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).

Interrupts are only one aspect. Insns sent back to guest context for
retry (like AVX2 gathers would commonly do) are another afaict.

> Furthermore, with 
> SVM the GLA is not available for page faults (although that's fixable by 
> comparing GPAs).

I may not have enough context here, but is that true when multiple
linear addresses are mapped to the same physical page?

Jan



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