xen/m2p: use GNTTABOP_unmap_and_replace to reinstate the original mapping

GNTTABOP_unmap_grant_ref unmaps a grant and replaces it with a 0
mapping instead of reinstating the original mapping.
Doing so separately would be racy.

To unmap a grant and reinstate the original mapping atomically we use
GNTTABOP_unmap_and_replace.
GNTTABOP_unmap_and_replace doesn't work with GNTMAP_contains_pte, so
don't use it for kmaps.  GNTTABOP_unmap_and_replace zeroes the mapping
passed in new_addr so we have to reinstate it, however that is a
per-cpu mapping only used for balloon scratch pages, so we can be sure that
it's not going to be accessed while the mapping is not valid.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Reviewed-by: David Vrabel <david.vrabel@citrix.com>
Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
CC: alex@alex.org.uk
CC: dcrisan@flexiant.com

[v1: Konrad fixed up the conflicts]
Conflicts:
	arch/x86/xen/p2m.c

1 files changed, 15 insertions, 6 deletions

diff --git a/arch/x86/xen/p2m.c b/arch/x86/xen/p2m.c
index 74672eeac881..0d4ec35895d4 100644
--- a/arch/x86/xen/p2m.c
+++ b/arch/x86/xen/p2m.c
@@ -161,6 +161,7 @@
 #include <asm/xen/page.h>
 #include <asm/xen/hypercall.h>
 #include <asm/xen/hypervisor.h>
+#include <xen/balloon.h>
 #include <xen/grant_table.h>
 
 #include "multicalls.h"
@@ -967,7 +968,10 @@ int m2p_remove_override(struct page *page,
         if (kmap_op != NULL) {
                 if (!PageHighMem(page)) {
                         struct multicall_space mcs;
-                        struct gnttab_unmap_grant_ref *unmap_op;
+                        struct gnttab_unmap_and_replace *unmap_op;
+                        struct page *scratch_page = get_balloon_scratch_page();
+                        unsigned long scratch_page_address = (unsigned long)
+                                __va(page_to_pfn(scratch_page) << PAGE_SHIFT);
 
                         /*
                          * It might be that we queued all the m2p grant table
@@ -990,20 +994,25 @@ int m2p_remove_override(struct page *page,
                         }
 
                         mcs = xen_mc_entry(
-                                        sizeof(struct gnttab_unmap_grant_ref));
+                                        sizeof(struct gnttab_unmap_and_replace));
                         unmap_op = mcs.args;
                         unmap_op->host_addr = kmap_op->host_addr;
+                        unmap_op->new_addr = scratch_page_address;
                         unmap_op->handle = kmap_op->handle;
-                        unmap_op->dev_bus_addr = 0;
 
                         MULTI_grant_table_op(mcs.mc,
-                                        GNTTABOP_unmap_grant_ref, unmap_op, 1);
+                                        GNTTABOP_unmap_and_replace, unmap_op, 1);
 
                         xen_mc_issue(PARAVIRT_LAZY_MMU);
 
-                        set_pte_at(&init_mm, address, ptep,
-                                        pfn_pte(pfn, PAGE_KERNEL));
+                        mcs = __xen_mc_entry(0);
+                        MULTI_update_va_mapping(mcs.mc, scratch_page_address,
+                                        pfn_pte(page_to_pfn(get_balloon_scratch_page()),
+                                        PAGE_KERNEL_RO), 0);
+                        xen_mc_issue(PARAVIRT_LAZY_MMU);
+
                         kmap_op->host_addr = 0;
+                        put_balloon_scratch_page();
                 }
         }