From da86f8cbad30ed3819ee5fd19a3b19291459c768 Mon Sep 17 00:00:00 2001 From: Alberto Garcia Date: Sat, 18 Jan 2020 20:09:28 +0100 Subject: [PATCH] qcow2: Use bs->bl.request_alignment when updating an L1 entry When updating an L1 entry the qcow2 driver writes a (512-byte) sector worth of data to avoid a read-modify-write cycle. Instead of always writing 512 bytes we should follow the alignment requirements of the storage backend. (the only exception is when the alignment is larger than the cluster size because then we could be overwriting data after the L1 table) Signed-off-by: Alberto Garcia Message-id: 71f34d4ae4b367b32fb36134acbf4f4f7ee681f4.1579374329.git.berto@igalia.com Reviewed-by: Max Reitz Signed-off-by: Max Reitz --- block/qcow2-cluster.c | 25 +++++++++++++++---------- 1 file changed, 15 insertions(+), 10 deletions(-) diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c index 0384fb2339..1947f13a2d 100644 --- a/block/qcow2-cluster.c +++ b/block/qcow2-cluster.c @@ -216,26 +216,31 @@ static int l2_load(BlockDriverState *bs, uint64_t offset, } /* - * Writes one sector of the L1 table to the disk (can't update single entries - * and we really don't want bdrv_pread to perform a read-modify-write) + * Writes an L1 entry to disk (note that depending on the alignment + * requirements this function may write more that just one entry in + * order to prevent bdrv_pwrite from performing a read-modify-write) */ -#define L1_ENTRIES_PER_SECTOR (512 / 8) int qcow2_write_l1_entry(BlockDriverState *bs, int l1_index) { BDRVQcow2State *s = bs->opaque; - uint64_t buf[L1_ENTRIES_PER_SECTOR] = { 0 }; int l1_start_index; int i, ret; + int bufsize = MAX(sizeof(uint64_t), + MIN(bs->file->bs->bl.request_alignment, s->cluster_size)); + int nentries = bufsize / sizeof(uint64_t); + g_autofree uint64_t *buf = g_try_new0(uint64_t, nentries); - l1_start_index = l1_index & ~(L1_ENTRIES_PER_SECTOR - 1); - for (i = 0; i < L1_ENTRIES_PER_SECTOR && l1_start_index + i < s->l1_size; - i++) - { + if (buf == NULL) { + return -ENOMEM; + } + + l1_start_index = QEMU_ALIGN_DOWN(l1_index, nentries); + for (i = 0; i < MIN(nentries, s->l1_size - l1_start_index); i++) { buf[i] = cpu_to_be64(s->l1_table[l1_start_index + i]); } ret = qcow2_pre_write_overlap_check(bs, QCOW2_OL_ACTIVE_L1, - s->l1_table_offset + 8 * l1_start_index, sizeof(buf), false); + s->l1_table_offset + 8 * l1_start_index, bufsize, false); if (ret < 0) { return ret; } @@ -243,7 +248,7 @@ int qcow2_write_l1_entry(BlockDriverState *bs, int l1_index) BLKDBG_EVENT(bs->file, BLKDBG_L1_UPDATE); ret = bdrv_pwrite_sync(bs->file, s->l1_table_offset + 8 * l1_start_index, - buf, sizeof(buf)); + buf, bufsize); if (ret < 0) { return ret; }