Re: [Xen-devel] [RFC 21/22] xen/arm: p2m: Re-implement p2m_set_mem_access using p2m_{set, get}_entry

[Tamas K Lengyel <tamas@xxxxxxxxxxxxx>]

On Mon, Aug 1, 2016 at 10:15 AM, Julien Grall <julien.grall@xxxxxxx> wrote:
>
>
> On 01/08/16 16:59, Tamas K Lengyel wrote:
>>
>> On Mon, Aug 1, 2016 at 9:40 AM, Julien Grall <julien.grall@xxxxxxx> wrote:
>> IMHO we should just pause the domain while mem_access permissions are
>> being changed. On x86 it is not necessary as the mem_access
>> bookkeeping is kept in the ept pte entries but since on ARM the two
>> things are disjoint it is required. Even on x86 though - while it's
>> not strictly necessary - I think it would be a good idea to just pause
>> the domain as from a user perspective it would be more intuitive then
>> keeping the vCPUs running.
>
>
> The problem is not because of the bookkeeping, but how the TLBs work. I
> never mentioned the radix tree in my previous mail because storing the
> access in the entry will not help here. The entry may have been updated but
> the TLBs not yet invalidated.
>
> x86 does not seem to be affected because if the mem access check fail, the
> instruction is replayed (see hvm_hap_nested_page_fault). This is exactly the
> solution I suggested, which is IHMO the best.

I see. Yes, indeed that sounds like the route to take here.

>
> I don't think pausing all the vCPUs of a domain is a good solution, the
> overhead will be too high for modifying only one page (see
> p2m_mem_access_check for instance).
>

IMHO the overhead of pausing the domain for setting mem_access
permissions is not critical as it is done only on occasion. We can
certainly leave the default behavior like this but I'll probably also
introduce a new XENMEM_access_op_set_access_sync op that will pause
the domain for the duration of the op.

Tamas

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
https://lists.xen.org/xen-devel