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

[Jan Beulich <jbeulich@xxxxxxxx>]

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().

> +    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 ...

> +    /*
> +     * 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;

> +    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.

> +            foreign_page = get_page_from_gfn(d, gfn_list[i], &p2mt, 
> P2M_ALLOC);
> +
> +            if ( unlikely(!(p2m_to_mask(p2mt) & valid_mask)) && foreign_page 
> )
> +            {
> +                put_page(foreign_page);
> +                foreign_page = NULL;
> +            }
> +            if ( unlikely(!foreign_page) )
> +            {
> +                gdprintk(XENLOG_WARNING,
> +                         "Error accessing foreign gfn %" PRI_gfn "\n",
> +                         gfn_list[i]);
> +                rc = -EINVAL;
> +                copy.nr_frames -= i;
> +                guest_handle_add_offset(copy.frame_list, i);
> +                goto out;
> +            }
> +
> +            foreign_mfn = page_to_mfn(foreign_page);
> +
> +            /* A page is dirtied when it's being copied to. */
> +            if ( direction == XENMEM_foreigncopy_to )
> +                paging_mark_dirty(d, foreign_mfn);
> +
> +            foreign = map_domain_page(foreign_mfn);
> +            if ( direction == XENMEM_foreigncopy_from )
> +                rc = copy_to_guest(copy.buffer, foreign, PAGE_SIZE);
> +            else
> +                rc = copy_from_guest(foreign, copy.buffer, PAGE_SIZE);

You cannot validly write to the page without holding a PGT_writable ref.
Else you might overwrite a page table or a descriptor table in a PV guest.

Once again - can you please make sure you have addressed earlier review
comments, before sending a new version? I did point this out before.

> +            unmap_domain_page(foreign);
> +            put_page(foreign_page);
> +
> +            if ( unlikely(rc) )
> +            {
> +                gdprintk(XENLOG_WARNING,
> +                         "Error %d copying gfn %" PRI_gfn "\n",
> +                         -rc, gfn_list[i]);

Why "-rc"? (See other log messages including error codes.)

> +                copy.nr_frames -= i;
> +                guest_handle_add_offset(copy.frame_list, i);
> +                goto out;
> +            }
> +
> +            guest_handle_add_offset(copy.buffer, PAGE_SIZE);
> +        }
> +
> +        copy.nr_frames -= todo;
> +        guest_handle_add_offset(copy.frame_list, todo);

Don't you need to also update copy.buffer?

> @@ -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".

> --- a/xen/include/public/memory.h
> +++ b/xen/include/public/memory.h
> @@ -740,7 +740,49 @@ 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.
> + * As this call requires target access and guest with target access won't be
> + * compat guests supported for compat guests this is not implemented.

As before - I question this. You simply can't know. (I'm also struggling with
wording / grammar.)

> + */
> +#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;
> +
> +    /*
> +     * IN/OUT
> +     *
> +     * Frames to be copied.
> +     * On output on error updated to point to first frame unhandled.

Is "on error" really correct / meaningful? The field can be updated at
any intermediate point, when a continuation is scheduled. Perhaps:

     * On output:
     *  - on error updated to point to first frame which couldn't be handled,
     *  - on success undefined.

Along these lines for nr_frames then as well (if needed at all, seeing
that it could as well be undefined in both cases, as the information is
redundant with the frame_list update).

> +     */
> +    XEN_GUEST_HANDLE(xen_pfn_t) frame_list;
> +
> +    /*
> +     * IN/OUT
> +     *
> +     * Userspace buffer to read/write from.

s/Userspace/Guest/ ?

Also still no mention of when / how this field is updated.

> +     */
> +    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.

Jan