Re: [Xen-devel] [PATCH v2 00/25] arm/altp2m: Introducing altp2m to ARM.

[Sergej Proskurin <proskurin@xxxxxxxxxxxxx>]

Hi Julien,


On 08/03/2016 04:08 PM, Julien Grall wrote:
> Hello Sergej,
>
> Please try to reply to all when answering on the ML. Otherwise the
> answer may be delayed/lost.
>

Right, sorry.

> On 03/08/16 13:45, Sergej Proskurin wrote:
>> The interesting part about #VE is that it allows to handle certain
>> violations (currently limited to EPT violations -- future
>> implementations might introduce also further violations) inside of the
>> guest, without the need to explicitly trap into the VMM. Thus, #VE allow
>> switching of different memory views in-guest. Because of this, I also
>> agree that event channels would suffice in our case, since we do not
>> have sufficient hardware support on ARM and would need to trap into the
>> VMM anyway.
>
> The cost of doing an hypercall on ARM is very small compare to x86
> (~1/3 of the number of x86 cycles) because we don't have to save all
> the state every time. So I am not convinced by the argument of
> limiting the number of trap to the hypervisor and allow a guest to
> play with altp2m on ARM.
>

I am not saying that we will be able to handle traps inside of the guest
-- simply because we don't have sufficient hardware support on ARM. What
I am trying to say is that an emulation of #VE is not required at this
point but could be implemented in the future by means of event channels,
if required.

Best regards,
~Sergej


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