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

[Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>]

On 04/28/2018 09:13 AM, Razvan Cojocaru 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). 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.
The only surefire way we have here is to single-step with the monitor 
trap flag set and with an unrestricted altp2m - or similar. We're doing 
something like that with XenServer:
https://github.com/xenserver/xen.pg/blob/XS-7.1.x/master/xen-reexecute-instn-under-monitor-trap.patch

But IIRC that patch had basically been nacked at the time, and I don't 
see a nice way of making the same logic work with alt2pm _without_ 
having vm_events triggered (which is what this optimization is about), 
due to, mainly, the fact that hostp2m changes propagate into altp2ms.

Thanks,
Razvan

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel