Re: Followup: HVMOP_altp2m_vcpu_enable_notify code example usage

[Tamas K Lengyel <tamas.k.lengyel@gmail.com>]

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On Thu, Oct 27, 2016 at 12:45 AM, Andrew Cooper <andrew.cooper3@citrix.com>
wrote:

> On 26/10/2016 20:37, Matt Leinhos wrote:
> > A while ago there was a thread by the same name in this group (see
> > https://lists.xen.org/archives/html/xen-devel/2016-08/msg02591.html).
> >
> > I've looked through the thread and did not see a resolution: do we have
> > an example handler for #VE - preferably on x86_64? I haven't found such
> > a handler in the xen code or through searches.
> >
> > I am attempting to use the altp2m API from a domU and need to handle #VE.
>
> A #VE handler is only interesting for a domU, knowing it is running
> under an altp2m-capable hypervisor.
>
> The hypervisor itself won't ever receive #VE from real hardware, which
> is why there is no example code in Xen.
>

Well, unless Xen is running in a nested setting ;)


>
> I don't know if there is any public example code, but a #VE handler in
> domU is intended to work very similarly to a plain #PF handler.  #VE
> specifically means "there is an EPT translation but the access failed
> for permission reasons, and the guest has elected to handle this fault
> itself".  It is then up to the domU kernel to decide what to do; whether
> to terminate the process/thread which cause the violation, or to alter
> the permissions and rerun the instruction.
>

Correct. The guest itself needs to issue the appropriate HVMOP hypercalls
to enable #VE and register the memory location where the trap info should
be stored at. The only caveat is that only permission faults in altp2m view
1-10 will generate a #VE in the domU, view 0 won't (at least when it is
emulated on hw without real #VE support).

Tamas

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