Re: [Xen-devel] [PATCH v1] x86/hvm: Generic instruction re-execution mechanism for execute faults

[Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>]

On 11/27/18 1:59 PM, Andrew Cooper wrote:
> On 27/11/2018 11:45, Razvan Cojocaru wrote:
>> On 11/27/18 1:32 PM, Roger Pau Monné wrote:
>>> Would it be possible to add some kind of flag to the emulator to
>>> signal whether p2m restrictions should be enforced/ignored?
>>> hvmemul_acquire_page seems like a suitable place, but I'm not that
>>> familiar with the emulator.
>>>
>>> Then you could generate vm events from the emulator itself, which
>>> AFAICT is the only way to handle this instruction execution issue.
>> I hope so, we'll definitely look into that.
> 
> FWIW, There is already a plan(tm).  It was discussed at least in part in
> Budapest.
> 
> The emulator needs to start honouring P2M permissions any generating
> vm_events.
> 
> Then, a vm_event response can reply saying "please emulate the
> instruction with this temporary change to the permissions", so
> write-ability to a read-only page can be granted at the discretion of
> the introspection agent.
> 
> That said, there is a huge amount of work required to make this happen,
> and I haven't had time to do a clear design yet.

Right, but for starters all we need is the ability to say
"hvm_emulate_one_vm_event(bool honour_page_walk_faults, bool
honor_gla_faults)".

Then we just replace all callsites of hvm_emulate_one_vm_event() with
hvm_emulate_one_vm_event(false, false), and the one in
p2m_mem_access_check() that we currently have with
hvm_emulate_one_vm_event(false, true).

Hopefully that makes sense. :)

Finer grained vm_event-based control is probably useful, but to the best
of my knowledge not currently (or in the near-medium future) necessary.


Thanks,
Razvan

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