Re: [Xen-devel] [PATCH 2/2] x86/hvm: honor guest's option when updating secondary system time for guest

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 10.07.14 at 13:00, <tim@xxxxxxx> wrote:
> At 07:18 +0800 on 08 Jul (1404800298), Feng Wu wrote:
>> In this patch, we add a new hypervisor which allows guests to enable
>> SMAP check when Xen is updating the secondary system time for guest.
>> The SMAP check for this update is disabled by default.
> 
> Again, it seems like we should never do SMAP checks here, since (a)
> this is a hypervisor operation, totally independent of whether the
> vcpu is in user mode or not, (b) this happens asynchronously to the
> guest so we can't raise a fault, an (c) this is explicitly _supposed_
> to access a user-space mapping. 

Not exactly: It is supposed to access a page that has a user space
mapping, but there's no implication that the update has to happen
through this user space mapping. In fact it is more likely for this not
to be the case, as I'd expect the user mode mapping to be r/o
while the mapping via which the update is to happen obviously has
to be r/w).

Jan


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