swiotlb-xen: maintain slab count properly

Generic swiotlb code makes sure to keep the slab count a multiple of the
number of slabs per segment. Yet even without checking whether any such
assumption is made elsewhere, it is easy to see that xen_swiotlb_fixup()
might alter unrelated memory when calling xen_create_contiguous_region()
for the last segment, when that's not a full one - the function acts on
full order-N regions, not individual pages.

Align the slab count suitably when halving it for a retry. Add a build
time check and a runtime one. Replace the no longer useful local
variable "slabs" by an "order" one calculated just once, outside of the
loop. Re-use "order" for calculating "dma_bits", and change the type of
the latter as well as the one of "i" while touching this anyway.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/dc054cb0-bec4-4db0-fc06-c9fc957b6e66@suse.com
Signed-off-by: Juergen Gross <jgross@suse.com>

diff --git a/drivers/xen/swiotlb-xen.c b/drivers/xen/swiotlb-xen.c
index dbb18dc..08fc694 100644 (file)
--- a/drivers/xen/swiotlb-xen.c
+++ b/drivers/xen/swiotlb-xen.c
@@ -106,27 +106,26 @@ static int is_xen_swiotlb_buffer(struct device *dev, dma_addr_t dma_addr)
 
 static int xen_swiotlb_fixup(void *buf, unsigned long nslabs)
 {
-       int i, rc;
-       int dma_bits;
+       int rc;
+       unsigned int order = get_order(IO_TLB_SEGSIZE << IO_TLB_SHIFT);
+       unsigned int i, dma_bits = order + PAGE_SHIFT;
        dma_addr_t dma_handle;
        phys_addr_t p = virt_to_phys(buf);
 
-       dma_bits = get_order(IO_TLB_SEGSIZE << IO_TLB_SHIFT) + PAGE_SHIFT;
+       BUILD_BUG_ON(IO_TLB_SEGSIZE & (IO_TLB_SEGSIZE - 1));
+       BUG_ON(nslabs % IO_TLB_SEGSIZE);
 
        i = 0;
        do {
-               int slabs = min(nslabs - i, (unsigned long)IO_TLB_SEGSIZE);
-
                do {
                        rc = xen_create_contiguous_region(
-                               p + (i << IO_TLB_SHIFT),
-                               get_order(slabs << IO_TLB_SHIFT),
+                               p + (i << IO_TLB_SHIFT), order,
                                dma_bits, &dma_handle);
                } while (rc && dma_bits++ < MAX_DMA_BITS);
                if (rc)
                        return rc;
 
-               i += slabs;
+               i += IO_TLB_SEGSIZE;
        } while (i < nslabs);
        return 0;
 }
@@ -210,7 +209,7 @@ retry:
 error:
        if (repeat--) {
                /* Min is 2MB */
-               nslabs = max(1024UL, (nslabs >> 1));
+               nslabs = max(1024UL, ALIGN(nslabs >> 1, IO_TLB_SEGSIZE));
                bytes = nslabs << IO_TLB_SHIFT;
                pr_info("Lowering to %luMB\n", bytes >> 20);
                goto retry;
@@ -245,7 +244,7 @@ retry:
                memblock_free(__pa(start), PAGE_ALIGN(bytes));
                if (repeat--) {
                        /* Min is 2MB */
-                       nslabs = max(1024UL, (nslabs >> 1));
+                       nslabs = max(1024UL, ALIGN(nslabs >> 1, IO_TLB_SEGSIZE));
                        bytes = nslabs << IO_TLB_SHIFT;
                        pr_info("Lowering to %luMB\n", bytes >> 20);
                        goto retry;