x86, vmi: put a missing paravirt_release_pmd in pgd_dtor

Commit 6194ba6f ("x86: don't special-case
pmd allocations as much") made changes to the way we handle pmd allocations,
and while doing that it dropped a call to  paravirt_release_pd on the
pgd page from the pgd_dtor code path.

As a result of this missing release, the hypervisor is now unaware of the
pgd page being freed, and as a result it ends up tracking this page as a
page table page.

After this the guest may start using the same page for other purposes, and
depending on what use the page is put to, it may result in various performance
and/or functional issues ( hangs, reboots).

Since this release is only required for VMI, I now release the pgd page from
the (vmi)_pgd_free hook.
Signed-off-by: Alok N Kataria <akataria@vmware.com>
Acked-by: Jeremy Fitzhardinge <jeremy@goop.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Cc: <stable@kernel.org>

arch/x86/kernel/vmi_32.c

@@ -320,6 +320,16 @@ static void vmi_release_pmd(unsigned long pfn)
 	vmi_ops.release_page(pfn, VMI_PAGE_L2);
 }
 
+/*
+ * We use the pgd_free hook for releasing the pgd page:
+ */
+static void vmi_pgd_free(struct mm_struct *mm, pgd_t *pgd)
+{
+	unsigned long pfn = __pa(pgd) >> PAGE_SHIFT;
+
+	vmi_ops.release_page(pfn, VMI_PAGE_L2);
+}
+
 /*
  * Helper macros for MMU update flags.  We can defer updates until a flush
  * or page invalidation only if the update is to the current address space
@@ -762,6 +772,7 @@ static inline int __init activate_vmi(void)
 	if (vmi_ops.release_page) {
 		pv_mmu_ops.release_pte = vmi_release_pte;
 		pv_mmu_ops.release_pmd = vmi_release_pmd;
+		pv_mmu_ops.pgd_free = vmi_pgd_free;
 	}
 
 	/* Set linear is needed in all cases */