Re: [PATCH v5 12/16] xen: implement new foreign copy hypercall

[Jan Beulich <jbeulich@xxxxxxxx>]

On 15.06.2026 14:11, Frediano Ziglio wrote:
> On Mon, 15 Jun 2026 at 08:41, Jan Beulich <jbeulich@xxxxxxxx> wrote:
>> On 13.06.2026 23:47, Frediano Ziglio wrote:
>>> --- a/xen/common/memory.c
>>> +++ b/xen/common/memory.c
>>> @@ -1545,6 +1545,133 @@ 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 & ~1U )
>>> +        return -EINVAL;
>>> +
>>> +    direction = copy.flags & XENMEM_foreigncopy_direction;
>>> +
>>> +    if ( copy.nr_frames == 0 )
>>> +        return 0;
>>> +
>>> +    rc = rcu_lock_remote_domain_by_id(copy.domid, &d);
>>> +    if ( rc )
>>> +        return rc;
>>
>> Extending my v4 remark: How about a "fully foreign" copy? I.e. one between 
>> two
>> pages in two foreign domains? Getting me back also to the un-answered v4
>> question of mine as to MMUEXT_COPY_PAGE.
> 
> I really thought I replied to this but I cannot find it.
> MMUEXT_COPY_PAGE was the first attempt to do this but mmuext is
> designed for PV and extending was made the code confusing.

We discussed this on the x86 call, and yes - the major op being PV-only is
getting in the way here.

>> Further, as to the order of checks: I'm not going to insist on re-ordering, 
>> yet
>> I'd like to point out that copying 0 pages to/from a bad domid might better
>> yield an error.
> 
> Not strong about this, changed to return -EINVAL. Reordering after
> this change won't make much sense, -EINVAL is returned both for wrong
> flags or no frames.

Please don't - copying 0 frames with all other arguments correct is not an
error.

>>> +    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 i = 0; i < todo; i++ )
>>
>> Nit: "unsigned int" please (like you have it above).
> 
> Changed. Note that "long" is also not a type but a modifier. Would it
> be good to change both the above to "unsigned" instead for consistency
> with "long" ?

No, the way we spell types has historical reasons and isn't tied to the
exact lexical meaning of the keywords.

>>> +        {
>>> +            struct page_info *foreign_page;
>>> +            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
>>
>> What about, for example, p2m_ram_ro? Or p2m_ram_shared? Or p2m_grant_map_*?
>> Etc. Any artificial constraining wants justifying in the description and/or
>> mentioning in the public header.
> 
> The base of this was taken from migration code where there is such a check.
> I suppose that adding p2m_ram_ro (where available) won't hurt.

Just to mention, to avoid another round trip just because of this: p2m_ram_ro
has different meaning on x86 vs Arm/RISC-V.

> p2m_ram_shared I'm not sure but seems fine too.
> For p2m_grant_map_* it feels a bit a security issue to me. It would
> allow a guest to give access to pages of other domains. It's true that
> the current domain would have to have write access to this domain
> anyway but extend these permissions sounds something it should not be
> able to do.

It could copy the contents of the grant mapped page by other means. Why not
allow it in this new sub-op as well then?

Talking of security: When the page you copy to is owned by a PV guest, I
think you further need to obtain a PGT_writable type ref. (Of course it then
likely is easier to always do this, not just for PV.)

>>> @@ -2012,6 +2139,13 @@ long do_memory_op(unsigned long cmd, 
>>> XEN_GUEST_HANDLE_PARAM(void) arg)
>>>              start_extent);
>>>          break;
>>>
>>> +    case XENMEM_foreigncopy:
>>> +        if ( unlikely(start_extent) )
>>> +            return -EINVAL;
>>
>> Please address review comments (verbally or by code changes) before 
>> submitting
>> a new version. Here I had asked "Why make this different from other 
>> continuable
>> sub-ops?"
>>
> 
> There's already a comment in the same file for similar reason
> 
>     /*
>      * Limiting nr_frames at (UINT_MAX >> MEMOP_EXTENT_SHIFT) isn't ideal.  If
>      * it ever becomes a practical problem, we can switch to mutating
>      * xmar.{frame,nr_frames,frame_list} in guest memory.
>      */
> 
> so to avoid the doubt and possible future change I mutate the structure.
> Also I use the mutation to give more information to the caller, using
> "start_entent" won't allow this.

You'll want to mention this in the description and/or a code comment. It
wants to become clear that the inconsistency in behavior (with other sub-
ops) is deliberate rather than accidental.

>>> --- a/xen/include/public/memory.h
>>> +++ b/xen/include/public/memory.h
>>> @@ -740,7 +740,45 @@ struct xen_vnuma_topology_info {
>>>  typedef struct xen_vnuma_topology_info xen_vnuma_topology_info_t;
>>>  DEFINE_XEN_GUEST_HANDLE(xen_vnuma_topology_info_t);
>>>
>>> -/* Next available subop number is 29 */
>>> +/*
>>> + * Copy memory from/to a given domain.
>>> + */
>>> +#define XENMEM_foreigncopy 29
>>> +struct xen_foreigncopy {
>>> +    /* IN - The domain whose resource is to be copied. */
>>
>> There's still "resource" here, when this really is about RAM (memory) only,
>> not any other kind of resource.
>>
>>> +    domid_t domid;
>>> +
>>> +    /* IN - Flags. */
>>> +#define XENMEM_foreigncopy_from 0
>>> +#define XENMEM_foreigncopy_to 1
>>> +#define XENMEM_foreigncopy_direction 1
>>> +    uint16_t flags;
>>> +
>>> +    /*
>>> +     * IN
>>> +     *
>>> +     * As an IN parameter number of frames of the domain to be copied.
>>> +     */
>>> +    uint32_t nr_frames;
>>
>> This isn't just an input, as you update the field (and the handles below).
>> This property of fields wants reflecting here, so callers know that they (a)
>> can't re-use the struct on a subsequent call without re-initializing the
>> fields which may have changed, and (b) can't put the struct in r/o memory.
>>
> 
> Update comments:
> 
> /*
>  * Copy memory from/to a given domain.
>  */
> #define XENMEM_foreigncopy 29
> struct xen_foreigncopy {
>     /* IN - The domain whose memory is to be copied. */
>     domid_t domid;
> 
>     /* IN - Flags. */
> #define XENMEM_foreigncopy_from 0
> #define XENMEM_foreigncopy_to 1
> #define XENMEM_foreigncopy_direction 1
>     uint16_t flags;
> 
>     /*
>      * IN/OUT
>      *
>      * As an IN parameter number of frames of the domain to be copied.
>      * On output on error updated number of frames left.
>      */
>     uint32_t nr_frames;

This is updated not only on error, but also when encoding continuations.

>>> +    /*
>>> +     * IN
>>> +     *
>>> +     * Frames to be copied.
>>> +     */
>>> +    XEN_GUEST_HANDLE(xen_pfn_t) frame_list;
>>> +
>>> +    /*
>>> +     * IN/OUT
>>> +     *
>>> +     * Userspace buffer to read/write from.
>>> +     */
>>> +    XEN_GUEST_HANDLE(uint8) buffer;
>>
>> With these two handles, there continues to be a need to (explicitly) deal
>> with the compat case as well.
> 
> I don't agree with this. Domains having access to other domains are
> limited (like stub domains for Qemu) and won't be 32 bits today so why
> allow 32 bits guests if not ever used?

How do you know? Why shouldn't e.g. XTF be permitted to test this in all
possible modes? And even if all arguments end up in favor of "no compat
support", this then wants spelling out to make clear this wasn't an
oversight, but rather a conscious decision.

Jan