Xen project Mailing List

On Thu, Jun 25, 2015 at 12:48 PM, Ed White <edmund.h.white@xxxxxxxxx> wrote:

On 06/25/2015 06:40 AM, Razvan Cojocaru wrote:
> On 06/25/2015 03:44 PM, Lengyel, Tamas wrote:
>> On Wed, Jun 24, 2015 at 2:06 PM, Ed White <edmund.h.white@xxxxxxxxx
>> <mailto:edmund.h.white@xxxxxxxxx>> wrote:
>> On 06/24/2015 09:15 AM, Lengyel, Tamas wrote:
>> >> +bool_t p2m_set_altp2m_mem_access(struct domain *d, uint16_t idx,
>> >> + unsigned long pfn, xenmem_access_t
>> >> access)
>> >> +{
>> >>
>> >
>> > This function IMHO should be merged with p2m_set_mem_access and should be
>> > triggerable with the same memop (XENMEM_access_op) hypercall instead of
>> > introducing a new hvmop one.
>>
>> I think we should vote on this. My view is that it makes
>> XENMEM_access_op
>> too complicated to use.
>>
>> The two functions are not very long and share enough code that it would
>> justify merging. The only big change added is the copy from host->alt
>> when the entry doesn't exists in alt, and that itself is pretty self
>> contained. Let's see if we can get a third opinion on it..
>
> At first sight (I admit I'm rather late in the game and haven't had a
> chance to follow the series closely from the beginning), the two
> functions do seem to be mergeable (or at least the common code factored
> out in static helper functions).
>
> Also, if Ed's concern is that the libxc API would look unnatural if
> xc_set_mem_access() is used for both purposes, as far as I can tell the
> only difference could be a non-zero last altp2m parameter, so I agree
> with you that the less functions doing almost the same thing the better
> (I have been guilty of this in the past too, for example with my
> xc_enable_introspection() function ;) ).
>
> So I'd say, yes, if possible merge them.

So here are my reasons why I don't think we should merge the hypercalls,
in more detail:

Although the two hypercalls are similar, they are not identical. For one
thing, the existing hypercall can only be used cross-domain whereas the
altp2m one can be used cross-domain or intra-domain.

Fair point, the use of rcu_lock_live_remote_domain_by_id in the memaccess memop handler precludes it working for the intra-domain case. However, now that we have a valid use-case for it working when a domain applies restrictions on itself, it would be fine to change that to rcu_lock_domain_by_any_id. It has been just used as a sanity check. The code you are using in hvm.c could be abstracted as p2m_altp2m_sanity_check: "!is_hvm_domain(d) || !hvm_altp2m_supported() || !d->arch.altp2m_active" and ran when the altp2m field is non-zero to catch buggy tools.

Also, the existing
hypercall can be used to modify a range of pages and the new one can only
modify a single page, and that is intentional.

Please elaborate on this.

As I see it, the implementation in hvm.c would become a lot less clean,
and every direct user of the existing hypercall would have to change for
no good reason.

For 4.6 I reworked the entire vm_event/mem_access system, so that is already happening irrespective of altp2m. It's fine to add support for the altp2m field before 4.6 freezes.

Razvan's suggestion to merge the functions that implement the p2m changes
I'm more ambivalent about. Personally, I prefer not to have code that
contains lots of conditional logic, which would be the result, but I
don't feel that strongly about it.

Ed

Thanks,

Tamas

Re: [Xen-devel] [PATCH v2 09/12] x86/altp2m: add remaining support routines.