Re: [Xen-devel] [PATCH 2/2] altp2m: Implement p2m_get_mem_access for altp2m views

[Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>]

On 01/28/2016 06:40 PM, Lengyel, Tamas wrote:
> 
> 
> On Thu, Jan 28, 2016 at 9:32 AM, Razvan Cojocaru
> <rcojocaru@xxxxxxxxxxxxxxx <mailto:rcojocaru@xxxxxxxxxxxxxxx>> wrote:
> 
>     On 01/28/2016 05:58 PM, Lengyel, Tamas wrote:
>     >
>     >
>     > On Thu, Jan 28, 2016 at 8:20 AM, Razvan Cojocaru
>     > <rcojocaru@xxxxxxxxxxxxxxx <mailto:rcojocaru@xxxxxxxxxxxxxxx>
>     <mailto:rcojocaru@xxxxxxxxxxxxxxx
>     <mailto:rcojocaru@xxxxxxxxxxxxxxx>>> wrote:
>     >
>     >     On 01/28/2016 05:12 PM, Lengyel, Tamas wrote:
>     >     >
>     >     > On Jan 28, 2016 8:02 AM, "Razvan Cojocaru" 
> <rcojocaru@xxxxxxxxxxxxxxx <mailto:rcojocaru@xxxxxxxxxxxxxxx>
>     <mailto:rcojocaru@xxxxxxxxxxxxxxx <mailto:rcojocaru@xxxxxxxxxxxxxxx>>
>     >     > <mailto:rcojocaru@xxxxxxxxxxxxxxx
>     <mailto:rcojocaru@xxxxxxxxxxxxxxx> <mailto:rcojocaru@xxxxxxxxxxxxxxx
>     <mailto:rcojocaru@xxxxxxxxxxxxxxx>>>> wrote:
>     >     >>
>     >     >> On 01/28/2016 04:42 PM, Lengyel, Tamas wrote:
>     >     >> >
>     >     >> > On Jan 28, 2016 6:38 AM, "Jan Beulich" <JBeulich@xxxxxxxx
>     <mailto:JBeulich@xxxxxxxx> <mailto:JBeulich@xxxxxxxx
>     <mailto:JBeulich@xxxxxxxx>>
>     >     > <mailto:JBeulich@xxxxxxxx <mailto:JBeulich@xxxxxxxx>
>     <mailto:JBeulich@xxxxxxxx <mailto:JBeulich@xxxxxxxx>>>
>     >     >> > <mailto:JBeulich@xxxxxxxx <mailto:JBeulich@xxxxxxxx>
>     <mailto:JBeulich@xxxxxxxx <mailto:JBeulich@xxxxxxxx>>
>     >     <mailto:JBeulich@xxxxxxxx <mailto:JBeulich@xxxxxxxx>
>     <mailto:JBeulich@xxxxxxxx <mailto:JBeulich@xxxxxxxx>>>>> wrote:
>     >     >> >>
>     >     >> >> >>> On 27.01.16 at 21:06, <tlengyel@xxxxxxxxxxx
>     <mailto:tlengyel@xxxxxxxxxxx> <mailto:tlengyel@xxxxxxxxxxx
>     <mailto:tlengyel@xxxxxxxxxxx>>
>     >     > <mailto:tlengyel@xxxxxxxxxxx <mailto:tlengyel@xxxxxxxxxxx>
>     <mailto:tlengyel@xxxxxxxxxxx <mailto:tlengyel@xxxxxxxxxxx>>>
>     >     >> > <mailto:tlengyel@xxxxxxxxxxx
>     <mailto:tlengyel@xxxxxxxxxxx> <mailto:tlengyel@xxxxxxxxxxx
>     <mailto:tlengyel@xxxxxxxxxxx>>
>     >     <mailto:tlengyel@xxxxxxxxxxx <mailto:tlengyel@xxxxxxxxxxx>
>     <mailto:tlengyel@xxxxxxxxxxx <mailto:tlengyel@xxxxxxxxxxx>>>>> wrote:
>     >     >> >> > --- a/xen/arch/x86/mm/p2m.c
>     >     >> >> > +++ b/xen/arch/x86/mm/p2m.c
>     >     >> >> > @@ -1572,7 +1572,9 @@ void
>     p2m_mem_access_emulate_check(struct
>     >     > vcpu *v,
>     >     >> >> >          bool_t violation = 1;
>     >     >> >> >          const struct vm_event_mem_access *data =
>     >     &rsp->u.mem_access;
>     >     >> >> >
>     >     >> >> > -        if ( p2m_get_mem_access(v->domain,
>     _gfn(data->gfn),
>     >     >> > &access) == 0 )
>     >     >> >> > +        if ( p2m_get_mem_access(v->domain,
>     _gfn(data->gfn),
>     >     >> >> > +                               
>     altp2m_active(v->domain) ?
>     >     >> > vcpu_altp2m(v).p2midx : 0,
>     >     >> >> > +                                &access) == 0 )
>     >     >> >>
>     >     >> >> This looks to be a behavioral change beyond what title and
>     >     >> >> description say, and it's not clear whether that's
>     actually the
>     >     >> >> behavior everyone wants.
>     >     >> >
>     >     >> > I'm fairly comfident its exactly the expected behavior
>     when one
>     >     uses
>     >     >> > mem_access in altp2m tables and emulation. Right now because
>     >     the lack of
>     >     >> > this AFAIK emulation would not work correctly with
>     altp2m. But
>     >     Razvan
>     >     >> > probably can chime in as he uses this path actively.
>     >     >>
>     >     >> I've done an experiment to see how much slower using altp2m
>     would
>     >     be as
>     >     >> compared to emulation - so I'm not a big user of the
>     feature, but
>     >     I did
>     >     >> find it cumbersome to have to work with two sets of APIs
>     (one for
>     >     what
>     >     >> could arguably be called the default altp2m view, i.e. the
>     regular
>     >     >> xc_set_mem_access(), and one for altp2m, i.e.
>     >     >> xc_altp2m_set_mem_access()). Furthermore, the APIs do not
>     currently
>     >     >> offer the same features (most notably,
>     xc_altp2m_get_mem_access() is
>     >     >> completely missing). I've mentioned this to Tamas while
>     initially
>     >     trying
>     >     >> to get it to work.
>     >     >>
>     >     >> Now, whether the behaviour I expect is what everyone
>     expects is, of
>     >     >> course, wide open to debate. But I think we can all agree
>     that the
>     >     >> altp2m interface can, and probably should, be improved.
>     >     >>
>     >     >
>     >     > There is that, but also, what is the exact logic behind
>     doing this
>     >     check
>     >     > before emulation? AFAIU emulation happens in response to a
>     vm_event so
>     >     > we should be fairly certain that this check succeeds as it just
>     >     verifies
>     >     > that indeed the permissions are restricted by mem_access in the
>     >     p2m (and
>     >     > with altp2m this should be the active one). But when is this
>     check
>     >     > normally expected to fail?
>     >
>     >     That check is important, please do not remove it. A vm_event
>     is sent
>     >     into userspace to our monitoring application, but the monitoring
>     >     application can actually remove the page restrictions before
>     replying,
>     >     so in that case emulation is pointless - there will be no more
>     page
>     >     faults for that instruction.
>     >
>     >
>     > I see, but then why would you reply with VM_EVENT_FLAG_EMULATE?
>     You know
>     > you removed the permission before sending the reply, so this
>     sounds like
>     > something specific to your application.
> 
>     It's cheap insurance that things go right. If there's some issue with
>     page rights, or some external tool somehow does an xc_set_mem_access(),
>     things won't go wrong.
> 
> 
> I can see this working for your application if you don't cache the
> mem_access permissions locally and you don't want to query for it before
> deciding to send the emulate flag in the response or not. Although, I
> think that would be the best way to go here.

Querying is out of the question, for obvious performance reasons. That's
why we've cached the registers in the vm_event request - we could have
not done that and instead just query them via libxc. But one small
decision like that and the monitored guest is running twice as slow.
This way, you can just set the emulate flag and have the hypervisor do
the right thing anyway, with no extra userspace <-> hypervisor roundtrips.

Caching might work, but then again that's extra work, memory used in the
application (in _each_ application, not just ours). So on one hand, we
have the current scenario where things can't go wrong and the solution
is in one place, vs. the other scenario, where each application needs to
solve the problem by doing tracking / caching / querying that the HV
does anyway in p2m, and pay with a possible guest crash or freeze for
failure.

>     And they will go wrong if Xen thinks it should
>     emulate the next instruction and the next instruction is not the one
>     that has caused the original fault.
> 
> 
> How could that happen? When the vCPU is resumed after the fault, isn't
> the same instruction guaranteed to be retried?

The instruction is the same, but if the page restrictions have been
lifted (somehow) and the EMULATE flag is still set, the original
instruction will run normally (because it won't trigger another page
fault). But the HV will still think that it needs to emulate the next
page fault, and so it will emulate whatever instruction causes the next
page fault (if it matches the emulate conditions).

>     I would think that benefits any
>     application.
> 
> 
> It's just a bit of an obscure exception. From an API perspective I would
> rather have Xen do what I tell it to do - in this case emulate - rather
> then it doing something else silently behind the scenes that you really
> only find out about if you read the code.

But the way the emulation code works now, it _can't_ emulate (see above
explanation). Emulation currently only happens as a result of a page
fault, and there will be no page fault if the page restriction are
lifted. I am thinking about a better way to achieve this, but until then
I think it's a good idea to keep the check in.

I hope I've been able to shed more light on this.


Thanks,
Razvan

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