Re: [Xen-devel] [RFC 15/16] xen/arm: traps: Don't inject a fault if the translation VA -> IPA fails

[Julien Grall <julien.grall@xxxxxxx>]

Hi Stefano,

On 21/05/16 15:51, Stefano Stabellini wrote:
> On Sat, 21 May 2016, Stefano Stabellini wrote:
> > On Thu, 5 May 2016, Julien Grall wrote:
> > > Based on ARM ARM (D4.5.3 in ARM DDI 0486A and B3.12.7 in ARM DDI 0406C.c),
> > > a Stage 1 translation error has priority over a Stage 2 translation error.
> > >
> > > Therefore gva_to_ipa can only fail if another vCPU is playing with the
> > > page table.
> > >
> > > Rather than injecting a custom fault, replay the instruction and let the
> > > processor injecting the correct fault.
> > >
> > > Signed-off-by: Julien Grall <julien.grall@xxxxxxx>
> > Couldn't a guest purposely cause a DoS in the hypervisor this way?
> Just double-checking. I am pretty sure it cannot, because the replayed
> instruction won't cause another hypervisor trap the second time around.
Before returning to the guest vCPU, Xen is handling any pending softirqs 
(see leave_hypervisor_tail). It might be possible to have the vCPU 
rescheduled.
So even if the replay cause another hypervisor trap, it will only impact 
its timeslice.
I will update the commit message to explain why it is not possible.

Regards,

--
Julien Grall

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