Re: [PATCH for-4.5 v10 15/19] xen/arm: Temporarily disable mem_access for hypervisor access

[Julien Grall <julien.grall@linaro.org>]

Hello Tamas,

On 26/09/2014 10:39, Tamas K Lengyel wrote:
> On Thu, Sep 25, 2014 at 6:19 PM, Julien Grall <julien.grall@linaro.org
> <mailto:julien.grall@linaro.org>> wrote:
>     I don't think that modifying temporary the permission is the right
>     thing to do because:
>              - p2m_set_mem_access is called 2 times which means 2 TLB
>     flush (and I'm not counting the table mapping), ie it's very slow
>              - The other VCPU of the guest are still running. So you may
>     not catch unwanted access.
>
>
> That is a problem. The only way around that I see is to pause the domain
> for the duration of this copy in case the mem_access permissions need to
> be disabled.

[..]

> So you mean only check the mem_access permissions when we failed to get
> the page. I'm not sure what you propose afterwards. If there is a
> mem_access restriction, just return an -errno? It would mean if a
> mem_access listener is trapped that page than the guest can't execute
> the hypercall. Since we would also want this system to be invisible to
> the guest, that I'm affraid is not a good approach.

The P2M is storing the type of the mapping. With this type you can 
easily know if the previous mapping was read/write and therefore know if 
the guest can effectively copy data to the page or not.

I don't see why we would need something more complicate as we want 
ignore mem_access for now.

Regards,

-- 
Julien Grall