[Xen-devel] 32-on-64 support in xen-unstable?

[Chris Lalancette <clalance@xxxxxxxxxx>]

Hello,
     I've been browsing through the preemptible pagetable stuff, and ran across
a piece of code that I don't understand or is buggy.  Looking at
arch/x86/mm.c:new_guest_cr3(), we have this code for 32-on-64 support:

    if ( is_pv_32on64_domain(d) )
    {
        okay = paging_mode_refcounts(d)
            ? 0 /* Old code was broken, but what should it be? */
            : mod_l4_entry(
                    __va(pagetable_get_paddr(curr->arch.guest_table)),
                    l4e_from_pfn(
                        mfn,
                        (_PAGE_PRESENT|_PAGE_RW|_PAGE_USER|_PAGE_ACCESSED)),
                    pagetable_get_pfn(curr->arch.guest_table), 0, 0, curr) == 0;
        if ( unlikely(!okay) )
        {
            MEM_LOG("Error while installing new compat baseptr %lx", mfn);
            return 0;
        }

However, looking at arch/x86/mm.c:mod_l4_entry(), it returns 0 on success and <
0 on error.  It seems to me that booting a 32-bit PV guest is always going to
fall into the !okay check on success, when it really shouldn't.  Shouldn't this
be something like:

        okay = paging_mode_refcounts(d)
            ? 0 /* Old code was broken, but what should it be? */
            : !mod_l4_entry(
                    __va(pagetable_get_paddr(curr->arch.guest_table)),
                    l4e_from_pfn(
                        mfn,
                        (_PAGE_PRESENT|_PAGE_RW|_PAGE_USER|_PAGE_ACCESSED)),
                    pagetable_get_pfn(curr->arch.guest_table), 0, 0, curr) == 0;

Or am I just missing something?  (NOTE: this still leaves open the possibility
for failure if mod_l4_entry() returns EINTR or EAGAIN, but maybe we don't have
to worry about that here?)

Thanks,
-- 
Chris Lalancette

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