recent changes to writable page tables on -unstable and xend

[Kip Macy <kip.macy@gmail.com>]

Any thought on which of the recent writable pagetable changes could be
causing an update to fail where it succeeded on xen 6 days ago?

(XEN) DOM5: (file=mm.c, line=2834) ptwr: Could not update pte at fe2ffc04
Timer ISR/1: Time went backwards: delta=-69992000 cpu_delta=1780008000 shadow=
1510880000000 off=9000 processed=1510950000000 cpu_processed=1509100000000
 0: 1510950000000
 1: 1509100000000

In my current changeset this comes from:

    /* Finally, make the p.t. page writable by the guest OS. */
    l1e_add_flags(&pte, _PAGE_RW);
    if ( unlikely(__copy_to_user(&linear_pg_table[addr>>PAGE_SHIFT],
                                 &pte, sizeof(pte))) )
    {
        MEM_LOG("ptwr: Could not update pte at %p", (unsigned long *)
                &linear_pg_table[addr>>PAGE_SHIFT]);
        /* Toss the writable pagetable state and crash. */


I've added show_guest_stack to determine that I'm in the process of
mapping a page read-write in order for it to be zeroed.

This isn't an emergency as disabling writable pagetables allows me to
get running again.


On -testing xend always reaps a domain immediately after it has
crashed. On -unstable up until today it would wait to reap the guest
until I ran 'xm list'. Now it appears that I have to explicitly run xm
destroy on a guest even after it has crashed.