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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 03/06/2026 2:39 pm, Jan Beulich wrote:
> On 03.06.2026 15:05, Frediano Ziglio wrote:
>> +    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?

Irrespective of what the compiler thinks or may do, Eclair will hard
reject it because unlikely() is a macro.

~Andrew