Re: [Xen-devel] [PATCH 2/2] xen: Introduce VM_EVENT_FLAG_SET_EIP

[Tamas K Lengyel <tamas@xxxxxxxxxxxxx>]

On Wed, Sep 16, 2015 at 12:12 PM, Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx> wrote:

On 09/16/2015 06:57 PM, Tamas K Lengyel wrote:
>
>
> On Tue, Sep 15, 2015 at 5:19 AM, Razvan Cojocaru
> <rcojocaru@xxxxxxxxxxxxxxx <mailto:rcojocaru@xxxxxxxxxxxxxxx>> wrote:
>
>Â Â ÂA previous version of this patch dealing with support for skipping
>Â Â Âthe current instruction when a vm_event response requested it
>Â Â Âcomputed the instruction length in the hypervisor, adding non-trivial
>Â Â Âcode dependencies. This patch allows a userspace vm_event client to
>Â Â Âsimply request that the guest's EIP is set to an arbitary value,
>Â Â Âcomputed by the introspection application.
>
>
> So in my opinion this patch introduces a feature that is not strictly
> tied to emulation related vm_event paths. I could use this feature to
> update the instruction pointer any time we respond to a vm_event and
> furthermore, it may be benefitial to expand the scope of which registers
> can be updated this way. For example, I have tools that update not just
> the instruction pointer but also the stack pointer and registers used to
> pass function inputs. Since we already send a snapshot of select
> registers to the user with each event, we could introduce a response
> flag that indicates that all registers included in that snapshot should
> be set to the values sent back by the user. The user then could choose
> which registers need to be updated in bulk.
>
> What do you think?

Hello Tamas, thanks for the reply!

Yes, I thought it might come up that this doesn't have to be
emulation-specific, but thought I'd hitch it there since I've assumed
that at the moment this is the only case where it's actually used.

I have nothing in principle against having a SET_REGISTERS flag instead
of a SET_EIP one, but I am unsure of how (and where) that would be best
implemented. What do you have in mind? A handler similar to void
vm_event_register_write_resume() where we set these registers for the
respective vcpu? Is this even possible at vm_event response handling time?

No, that function falls under a switch on rsp.reason, for which we have a 1:1 unofficial and not really enforced rule to match the type of event that was sent. This should fall under a flag on rsp.flags and be handled similar to how vm_event_toggle_singlestep is.

TamasÂ

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