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Subject[PATCH v4 2/2] xen/m2p: use GNTTABOP_unmap_and_replace to reinstate the original mapping
Date
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>
CC: alex@alex.org.uk
CC: dcrisan@flexiant.com
---
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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