[Xen-devel] [PATCH v4 2/2] xen/m2p: use GNTTABOP_unmap_and_replace to reinstate the original mapping

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

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@xxxxxxxxxxxxx>
Reviewed-by: David Vrabel <david.vrabel@xxxxxxxxxx>
CC: alex@xxxxxxxxxxx
CC: dcrisan@xxxxxxxxxxxx
---
 arch/x86/xen/p2m.c   |   22 +++++++++++++++-------
 drivers/xen/gntdev.c |   11 ++---------
 2 files changed, 17 insertions(+), 16 deletions(-)

diff --git a/arch/x86/xen/p2m.c b/arch/x86/xen/p2m.c
index 95fb2aa..0d4ec35 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
@@ -990,21 +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));
-                       __flush_tlb_single(address);
+                       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();
                }
        }
 
diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c
index 3c8803f..51f4c95 100644
--- a/drivers/xen/gntdev.c
+++ b/drivers/xen/gntdev.c
@@ -270,19 +270,12 @@ static int map_grant_pages(struct grant_map *map)
                 * with find_grant_ptes.
                 */
                for (i = 0; i < map->count; i++) {
-                       unsigned level;
                        unsigned long address = (unsigned long)
                                pfn_to_kaddr(page_to_pfn(map->pages[i]));
-                       pte_t *ptep;
-                       u64 pte_maddr = 0;
                        BUG_ON(PageHighMem(map->pages[i]));
 
-                       ptep = lookup_address(address, &level);
-                       pte_maddr = arbitrary_virt_to_machine(ptep).maddr;
-                       gnttab_set_map_op(&map->kmap_ops[i], pte_maddr,
-                               map->flags |
-                               GNTMAP_host_map |
-                               GNTMAP_contains_pte,
+                       gnttab_set_map_op(&map->kmap_ops[i], address,
+                               map->flags | GNTMAP_host_map,
                                map->grants[i].ref,
                                map->grants[i].domid);
                }
-- 
1.7.2.5


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