Re: [PATCH v6 12/16] xen: implement new foreign copy hypercall

[Jan Beulich <jbeulich@xxxxxxxx>]

On 23.06.2026 23:18, Frediano Ziglio wrote:
> On Tue, 23 Jun 2026 at 14:21, Jan Beulich <jbeulich@xxxxxxxx> wrote:
>> On 23.06.2026 12:55, Frediano Ziglio wrote:
>>> On Mon, 22 Jun 2026 at 11:34, Jan Beulich <jbeulich@xxxxxxxx> wrote:
>>>> On 19.06.2026 15:04, Frediano Ziglio wrote:
>>>>> --- a/xen/common/memory.c
>>>>> +++ b/xen/common/memory.c
>>>>> @@ -1545,6 +1545,139 @@ static int acquire_resource(
>>>>>      return rc;
>>>>>  }
>>>>>
>>>>> +/*
>>>>> + * The "noinline" qualifier avoids the compiler to create a large 
>>>>> function
>>>>> + * consuming quite a lot of stack.
>>>>> + */
>>>>> +static int noinline mem_foreigncopy(
>>>>> +    XEN_GUEST_HANDLE_PARAM(xen_foreigncopy_t) arg)
>>>>> +{
>>>>> +    struct domain *d, *const currd = current->domain;
>>>>> +    xen_foreigncopy_t copy;
>>>>> +    int rc, direction;
>>>>> +
>>>>> +    if ( copy_from_guest(&copy, arg, 1) )
>>>>> +        return -EFAULT;
>>>>> +
>>>>> +    if ( copy.flags & ~XENMEM_foreigncopy_direction )
>>>>> +        return -EINVAL;
>>>>> +
>>>>> +    direction = copy.flags & XENMEM_foreigncopy_direction;
>>>>> +
>>>>> +    rc = rcu_lock_remote_domain_by_id(copy.domid, &d);
>>>>
>>>> Iirc I did ask before why this isn't ..._by_any_id().
>>>
>>> I probably was confused by the question about MMUEXT and the 2 domains.
>>> There are different similar hypercalls (like the mentioned MMUEXT but
>>> also hypercalls to map foreign domain memory) that have this check
>>> (not the same domain). Any domain has, obviously, access to its own
>>> memory, so it should not have to use hypercall to access its own
>>> memory. If it does it looks like a mistake causing performance issues
>>> or an attempt to circumvent security; in either case you would like to
>>> avoid it.
>>
>> No. Self-grants are possible as well, for example, and for a good reason.
>> Allowing normally-remote operations on oneself helps with testing, for
>> example. It may also help avoid needing to special-case "self" in code
>> which needs to cover both cases.
> 
> But this is not a grant, it's a copy.

Sure, but the underlying principle is what matters. Plus you don't prevent
self-copy by using ..._by_id(), you only preclude the use of DOMID_SELF.

>>>>> +    if ( rc )
>>>>> +        return rc;
>>>>> +
>>>>> +    if ( copy.nr_frames == 0 )
>>>>> +    {
>>>>> +        rcu_unlock_domain(d);
>>>>> +        return 0;
>>>>> +    }
>>>>
>>>> Any reason this cannot also be "goto out"? The more that now that you have
>>>> moved this past the domid validity check, imo it should further move to ...
>>>
>>> The only reason was style and to avoid a memory copy, but it's not a
>>> hot case so I'll change to "goto out" (no strong about it).
>>>
>>>>> +    /*
>>>>> +     * Check we are allowed to map and access these foreign pages.
>>>>> +     */
>>>>> +    rc = xsm_map_gmfn_foreign(XSM_TARGET, currd, d);
>>>>> +    if ( rc )
>>>>> +        goto out;
>>>>
>>>> ... below here. Perhaps simply as
>>>>
>>>>     if ( rc || !copy.nr_frames )
>>>>         goto out;
>>>>
>>>
>>> I think this would be confusing with the above "Check we are allowed
>>> to map and access these foreign pages" comment.
>>> Are you okay with just the change above to "goto out" ?
>>
>> I do want the order adjusted as indicated. I won't insist on (but I would
>> prefer) folding both if()-s.
>>
> 
> What about
> 
>     /*
>      * Check we are allowed to map and access these foreign pages.
>      */
>     rc = xsm_map_gmfn_foreign(XSM_TARGET, currd, d);
>     if ( rc )
>         goto out;
> 
>     while ( copy.nr_frames )
>     {
>         /*
>          * Arbitrary size.  Not too much stack space, and a reasonable stride
>          * for continuation checks.
>          */

That's fine.

>>>>> +    do {
>>>>> +        /*
>>>>> +         * Arbitrary size.  Not too much stack space, and a reasonable 
>>>>> stride
>>>>> +         * for continuation checks.
>>>>> +         */
>>>>> +        xen_pfn_t gfn_list[32];
>>>>> +        unsigned int todo = MIN(ARRAY_SIZE(gfn_list), copy.nr_frames);
>>>>> +
>>>>> +        rc = -EFAULT;
>>>>> +        if ( copy_from_guest(gfn_list, copy.frame_list, todo) )
>>>>> +            goto out;
>>>>> +
>>>>> +        for ( unsigned int i = 0; i < todo; i++ )
>>>>> +        {
>>>>> +            struct page_info *foreign_page;
>>>>> +            mfn_t foreign_mfn;
>>>>> +            void *foreign;
>>>>> +            p2m_type_t p2mt;
>>>>> +            const unsigned long valid_mask =
>>>>> +#ifdef CONFIG_X86
>>>>> +                p2m_to_mask(p2m_ram_rw) | p2m_to_mask(p2m_ram_logdirty);
>>>>> +#else
>>>>> +                p2m_to_mask(p2m_ram_rw);
>>>>> +#endif
>>>>
>>>> The set of permitted types didn't change, yet a justification for the 
>>>> resulting
>>>> limitation also didn't appear.
>>>>
>>>
>>> Yes, that's missing, indeed.
>>> Should the set of types be different for reading and writing? For
>>> instance do not allow writing to read-only memory?
>>
>> Of course.
>>
>>> Given that it looks like different architectures have different
>>> meanings and definitions for these constants, should it not be better
>>> to define some new constants for this specific usage? For instance
>>> P2M_READ_TYPES and P2M_WRITE_TYPES?
>>
>> Perhaps, yes. The suggested names look overly generic to me, though.
> 
> I suppose P2M_READABLE_TYPES and P2M_WRITABLE_TYPES are more correct
> but still too generic.
> P2M_EXPORTABLE_TYPES and P2M_IMPORTABLE_TYPES ?

First: Do you foresee uses of those constants anywhere else? If not (I
don't), tie the names to this particular operation. That'll make them
entirely non-generic.

>>>>> @@ -2012,6 +2145,18 @@ long do_memory_op(unsigned long cmd, 
>>>>> XEN_GUEST_HANDLE_PARAM(void) arg)
>>>>>              start_extent);
>>>>>          break;
>>>>>
>>>>> +    case XENMEM_foreigncopy:
>>>>> +        /*
>>>>> +         * Instead of using "start_extent" we update the structure back,
>>>>> +         * we update it back in anyway to tell caller were the copy
>>>>> +         * stopped.
>>>>> +         */
>>>>> +        if ( unlikely(start_extent) )
>>>>> +            return -EINVAL;
>>>>
>>>> As before - please be precise with comments like this. We update it back 
>>>> also
>>>> when encoding a continuation. Perhaps instead "..., to indicate the point 
>>>> of
>>>> failure to the caller as well as to encode continuations without being
>>>> constrained by MEMOP_EXTENT_SHIFT".
>>>>
>>>
>>> What about (trying to include your suggestion, to be fixed for line length):
>>>
>>>         /*
>>>          * Instead of using "start_extent" for the continuation, we
>>> update the structure back,
>>>          * we update the xen_foreigncopy structure back, so we are not
>>> constrained
>>>          * by MEMOP_EXTENT_SHIFT.
>>>          * We copy it back also to tell the caller where the copy stopped.
>>>          */
>>
>> One of the things I take issue with (because it's hard to read that way,
>> at least for me) is the repeated use of "update ... back", effectively
>> saying the same things twice. The last sentence also wants disambiguating
>> towards the "stopped" possibly being a non-error situation as well.
>>
> 
> Changed to
> 
>         /*
>          * Instead of using "start_extent" for the continuation, we update
>          * the xen_foreigncopy structure back, so we are not constrained by
>          * MEMOP_EXTENT_SHIFT.
>          * We copy it back also to tell the caller where the copy stopped
>          * (either for error or because all frames were copied).
>          */

Thanks.

>>>>> +    XEN_GUEST_HANDLE(uint8) buffer;
>>>>> +};
>>>>
>>>> What was (again) left unaddressed is the question towards using GFNs on 
>>>> both
>>>> sides of the copy. This would eliminate the need for the flags field, taken
>>>> by a 2nd domid_t one then.
>>>>
>>>
>>> This was addressed in
>>> https://lists.xenproject.org/archives/html/xen-devel/2026-06/msg00567.html
>>
>> Well, yes, but not in a satisfactory way. Back channels tell me that you
>> actually got the same feedback already on internal review. Which makes it
>> all the more puzzling that you insist on doing it differently. Multiple
>> maintainers asking for the same thing may be an indication of something.
> 
> Not needing to have backchannel feedback, I already wrote that a
> similar approach was tried and made the code more complicated.

Even if indeed so: Yet at the same time more flexible.

> Both maintainers didn't comment on my replies so I assume they were
> fine with it.
> And you are failing to provide positive feedback.
> I asked (that one internally) for examples of guest buffers provided
> as frame numbers but I got no answer (or better the answer was more
> "currently there are not").
> Also note that the location of xen_foreigncopy_t structure is also
> provided using a guest pointer.
> I remember there were some discussions about ABI changes (2/3 years
> ago) to address this and other issues but I cannot see much progress.

And it's that (very slowly progressing effort) which made me ask. The
fewer virtual addresses we bake into new sub-ops, the better for that
effort. And no, that doesn't go as far as completely eliminating
handles (presently representing virtual addresses) - that needs to be
part of the new ABI.

To preempt the argument towards "fewer virtual addresses" not really
being true when changing from handle-to-uint8 to handle-to-pfn: The
former won't be able to express a buffer mapped contiguously in VA
space, but discontiguous in PA space. The latter will, simply be
avoiding buffer VAs in the first place (the array of frame numbers
can e.g. be placed in a dedicated hypercall argument area known to be
physically contiguous).

> That's why I say this is out of scope.

There's nothing scope related here. We're discussing how to shape the
new sub-op interface.

>>> and in minor way by
>>> https://lists.xenproject.org/archives/html/xen-devel/2026-06/msg00847.html.
>>> It was considered but more complicated and worse from a performance 
>>> perspective.
>>
>> Okay, performance-wise worse would of course be relevant. But that would
>> need supporting by numbers (for both PV and PVH Dom0, as the latter
>> incurs extra overhead for virtual-address-based hypercall buffer operands).
> 
> I'm more concerned about the PV case than PVH to be honest.

For your (immediate) internal purposes that may be fine, but PVH Dom0
more likely being the future, for upstream both need considering
equally.

Jan