IOCTL_PRIVCMD_MMAPBATCH on Xen 4.13.0

* IOCTL_PRIVCMD_MMAPBATCH on Xen 4.13.0
@ 2020-05-15 20:29 Manuel Bouyer
  2020-05-15 21:00 ` Andrew Cooper
  0 siblings, 1 reply; 16+ messages in thread
From: Manuel Bouyer @ 2020-05-15 20:29 UTC (permalink / raw)
  To: xen-devel

Hello,
NetBSD works as dom0 up to Xen 4.11. I'm trying to get it working
on 4.13.0. I added the support for gntdev operations,  but I'm stuck with
privcmd IOCTL_PRIVCMD_MMAPBATCH. It seems to work fine for PV and PVH domUs,
but with HVM domUs, MMU_NORMAL_PT_UPDATE returns -22 (EINVAL) and
qemu-dm dumps core (as expected; the page is not mapped).
Of course this works fine in 4.11

In the Xen kernel, I tracked it down to arch/x86/mm.c near line 2229,
in mod_l1_entry():
        /* Translate foreign guest address. */
        if ( cmd != MMU_PT_UPDATE_NO_TRANSLATE &&
             paging_mode_translate(pg_dom) )
        {
            p2m_type_t p2mt;
            p2m_query_t q = l1e_get_flags(nl1e) & _PAGE_RW ?
                            P2M_ALLOC | P2M_UNSHARE : P2M_ALLOC;

            page = get_page_from_gfn(pg_dom, l1e_get_pfn(nl1e), &p2mt, q);

            if ( p2m_is_paged(p2mt) )
            {
                if ( page )
                    put_page(page);
                p2m_mem_paging_populate(pg_dom, l1e_get_pfn(nl1e));
                return -ENOENT;
            }

            if ( p2mt == p2m_ram_paging_in && !page )
                return -ENOENT;

            /* Did our attempt to unshare fail? */
            if ( (q & P2M_UNSHARE) && p2m_is_shared(p2mt) )
            {
                /* We could not have obtained a page ref. */
                ASSERT(!page);
                /* And mem_sharing_notify has already been called. */
                return -ENOMEM;
            }

            if ( !page ) {
                gdprintk(XENLOG_WARNING, "translate but no page\n");
                return -EINVAL;
            }                        
            nl1e = l1e_from_page(page, l1e_get_flags(nl1e));
        }

the gdprintk() I added in the ( !page) case fires, so this is the
cause of the EINVAL.
Is it expected for a HVM domU ? If so, how should the dom0 code be
changed to get it working ? I failed to see where our code is different
from linux ...

-- 
Manuel Bouyer <bouyer@antioche.eu.org>
     NetBSD: 26 ans d'experience feront toujours la difference
--

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