Re: [PATCH v4 12/16] xen: implement new foreign copy hypercall

[Jan Beulich <jbeulich@xxxxxxxx>]

On 03.06.2026 15:05, Frediano Ziglio wrote:
> --- a/xen/common/memory.c
> +++ b/xen/common/memory.c
> @@ -1545,6 +1545,132 @@ static int acquire_resource(
>      return rc;
>  }
>  
> +/*
> + * The "noinline" qualifier avoid 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 ( !arch_acquire_resource_check(currd) )
> +        return -EACCES;

How is the new sub-op related to acquire-resource? And aren't you instead
lacking an XSM check?

> +    if ( copy_from_guest(&copy, arg, 1) )
> +        return -EFAULT;
> +
> +    if ( copy.flags & ~1u )

If a suffix is needed here in the first place, please use an upper-case one.
Misra only demands L suffixes to be upper-case, but iirc we decided that
then we want all suffixes that way.

> +        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);

Why not rcu_lock_domain_by_any_id()? IOW why would a self-copy need
prohibiting? 

> +    if ( rc )
> +        return rc;
> +
> +    /*
> +     * Check we are allowed to map and access these foreign pages.
> +     */
> +    rc = xsm_map_gmfn_foreign(XSM_TARGET, currd, d);
> +    if ( rc )
> +        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 i = 0; i < todo; i++ )
> +        {
> +            struct page_info *foreign_page;
> +            void *foreign;
> +            p2m_type_t p2mt;
> +
> +            foreign_page = get_page_from_gfn(d, gfn_list[i], &p2mt, 
> P2M_ALLOC);
> +
> +            if ( unlikely(p2mt != p2m_ram_rw
> +#ifdef CONFIG_X86
> +                 && p2mt != p2m_ram_logdirty
> +#endif
> +                 ) && foreign_page )

This is ugly formatting wise, and the use of unlikely() isn't very likely
to have the effect you intend: As long as the compiler can't translate the
&& expression to something involving only a single conditional branch,
which of the branches is it that is unlikely to be taken?

> +            {
> +                put_page(foreign_page);
> +                foreign_page = NULL;
> +            }
> +            if ( unlikely(!foreign_page) )
> +            {
> +                gdprintk(XENLOG_WARNING,
> +                         "Error accessing foreign mfn %" PRI_mfn "\n",
> +                         gfn_list[i]);

As per get_page_from_gfn() and gfn_list[] it's a GFN, not an MFN.

> +                rc = -EINVAL;
> +                copy.nr_frames -= i;
> +                guest_handle_add_offset(copy.frame_list, i);
> +                goto out;
> +            }
> +
> +            /* A page is dirtied when it's being copied to. */
> +            if ( direction == XENMEM_foreigncopy_to )
> +                paging_mark_dirty(d, page_to_mfn(foreign_page));
> +
> +            foreign = map_domain_page(page_to_mfn(foreign_page));
> +            if ( direction == XENMEM_foreigncopy_from )
> +                rc = copy_to_guest(copy.buffer, foreign, PAGE_SIZE);
> +            else
> +                rc = copy_from_guest(foreign, copy.buffer, PAGE_SIZE);
> +            unmap_domain_page(foreign);
> +            put_page(foreign_page);
> +
> +            if ( unlikely(rc) )
> +            {
> +                gdprintk(XENLOG_WARNING,
> +                         "Error copying to mfn %" PRI_mfn "\n", gfn_list[i]);

"to" isn't always correct here, and the problem wasn't with the GFN (not
MFN) anyway, but with the buffer.

> +                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);
> +
> +        if ( copy.nr_frames && hypercall_preempt_check() )
> +        {
> +            rc = hypercall_create_continuation(
> +                __HYPERVISOR_memory_op, "lh", XENMEM_foreigncopy, arg);
> +            goto out;
> +        }
> +    } while ( copy.nr_frames );
> +
> +    rc = 0;
> +
> + out:
> +    rcu_unlock_domain(d);
> +
> +    /* Update in all cases, it allows the caller to know how many
> +     * frames were successfully copied and the continuation to
> +     * continue correctly.
> +     */
> +    if ( copy_to_guest(arg, &copy, 1) )

Since you already used copy_from_guest() one the way in, __copy_to_guest()
will do here.

> +        rc = -EFAULT;
> +
> +    return rc;
> +}

Before looking at the implementation in yet more detail: This is quite a
bit of new code. Did you at least consider extending MMUEXT_COPY_PAGE
along the lines of what c6b8bdfe3b47 ("x86: extend mmu_update hypercall
to allow update of foreign pagetables") did, allowing two domains to be
specified in the foreigndom argument?

> @@ -2012,6 +2138,13 @@ 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;

Why make this different from other continuable sub-ops?

> --- 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. */
> +    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;
> +
> +    /*
> +     * 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;
> +};

Seeing the two handles - what about compat_memory_op()?

Jan