VMI bug fix.

[Alok Kataria]

Hi Tim, Stefan,

Below is a patch which i posted on LKML, this fixes a critical bug in
the VMI paravirt code. Without this patch users could see random hangs
or performance drops running a VMI kernel. 
Given that Ubuntu has PARAVIRT enabled by default, any 32bit kernel
running on VMware platform will see this problem.

This bug slipped in the kernel in 2.6.25 time frame. So the fix is
needed only for the Ubuntu Intrepid and Jaunty series.
Hardy is good in its current state.

Here is the link to the LKML discussion. 
http://lkml.org/lkml/2009/2/5/542

The patch is merged in Ingo's tip tree and should be merged in mainline
in the 29.rc-5 cycle. Commit details are
-----------------------------------------------------------------
commit 55a8ba4b7f76bebd7e8ce3f74c04b140627a1bad
Author: Alok Kataria <akataria at vmware.com>
Date:   Fri Feb 6 10:29:35 2009 -0800

    x86, vmi: put a missing paravirt_release_pmd in pgd_dtor
-----------------------------------------------------------------

Attaching it below too.
Please apply it to the Intrepid and Jaunty tree.

Thanks,
Alok
--

commit 55a8ba4b7f76bebd7e8ce3f74c04b140627a1bad
Author: Alok Kataria <akataria at vmware.com>
Date:   Fri Feb 6 10:29:35 2009 -0800

    x86, vmi: put a missing paravirt_release_pmd in pgd_dtor
    
    Commit 6194ba6ff6ccf8d5c54c857600843c67aa82c407 ("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 at vmware.com>
    Acked-by: Jeremy Fitzhardinge <jeremy at goop.org>
    Signed-off-by: Ingo Molnar <mingo at elte.hu>
    Cc: <stable at kernel.org>

diff --git a/arch/x86/kernel/vmi_32.c b/arch/x86/kernel/vmi_32.c
index 1d3302c..bef58b4 100644
--- a/arch/x86/kernel/vmi_32.c
+++ b/arch/x86/kernel/vmi_32.c
@@ -321,6 +321,16 @@ static void vmi_release_pmd(unsigned long pfn)
 }
 
 /*
+ * 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
  * (otherwise, there is no flush).  We must check against init_mm, since
@@ -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 */