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

[Tamas K Lengyel <tamas@xxxxxxxxxxxxx>]

On Tue, Sep 15, 2015 at 5:19 AM, Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx> wrote:

A previous version of this patch dealing with support for skipping
the current instruction when a vm_event response requested it
computed the instruction length in the hypervisor, adding non-trivial
code dependencies. This patch allows a userspace vm_event client to
simply request that the guest's EIP is set to an arbitary value,
computed 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?

Thanks,

Tamas

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