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

[Julien Grall <julien.grall@xxxxxxx>]

On 01/08/16 17:27, Tamas K Lengyel wrote:
> 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 am looking forward to see a patch.

> > 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.
Oh, you meant for a user app to set the memaccess. I guess it is fine. I 
mentioned p2m_mem_access_check because this function is supposed to be 
called often in the data/instruction abort handlers, so pausing a domain 
here would be a big overhead.
Regards,

--
Julien Grall

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