Avoid bounce buffers when QEMUIOVector elements are within previously registered bdrv_register_buf() buffers.
The idea is that emulated storage controllers will register guest RAM using bdrv_register_buf() and set the BDRV_REQ_REGISTERED_BUF on I/O requests. Therefore no blkio_map_mem_region() calls are necessary in the performance-critical I/O code path. This optimization doesn't apply if the I/O buffer is internally allocated by QEMU (e.g. qcow2 metadata). There we still take the slow path because BDRV_REQ_REGISTERED_BUF is not set. Signed-off-by: Stefan Hajnoczi <stefa...@redhat.com> --- block/blkio.c | 142 ++++++++++++++++++++++++++++++++++++++++++++++++-- 1 file changed, 139 insertions(+), 3 deletions(-) diff --git a/block/blkio.c b/block/blkio.c index cdcba1b63e..976d37705c 100644 --- a/block/blkio.c +++ b/block/blkio.c @@ -11,7 +11,10 @@ #include "qemu/osdep.h" #include <blkio.h> #include "block/block_int.h" +#include "exec/memory.h" +#include "exec/cpu-common.h" /* for qemu_ram_get_fd() */ #include "qapi/error.h" +#include "qemu/error-report.h" #include "qapi/qmp/qdict.h" #include "qemu/module.h" @@ -46,6 +49,9 @@ typedef struct { /* Can we skip adding/deleting blkio_mem_regions? */ bool needs_mem_regions; + + /* Are file descriptors necessary for blkio_mem_regions? */ + bool needs_mem_region_fd; } BDRVBlkioState; static void blkio_aiocb_complete(BlkioAIOCB *acb, int ret) @@ -226,6 +232,8 @@ static BlockAIOCB *blkio_aio_preadv(BlockDriverState *bs, int64_t offset, BlockCompletionFunc *cb, void *opaque) { BDRVBlkioState *s = bs->opaque; + bool needs_mem_regions = + s->needs_mem_regions && !(flags & BDRV_REQ_REGISTERED_BUF); struct iovec *iov = qiov->iov; int iovcnt = qiov->niov; BlkioAIOCB *acb; @@ -234,7 +242,7 @@ static BlockAIOCB *blkio_aio_preadv(BlockDriverState *bs, int64_t offset, acb = blkio_aiocb_get(bs, cb, opaque); - if (s->needs_mem_regions) { + if (needs_mem_regions) { if (blkio_aiocb_init_mem_region_locked(acb, bytes) < 0) { qemu_aio_unref(&acb->common); return NULL; @@ -258,6 +266,8 @@ static BlockAIOCB *blkio_aio_pwritev(BlockDriverState *bs, int64_t offset, { uint32_t blkio_flags = (flags & BDRV_REQ_FUA) ? BLKIO_REQ_FUA : 0; BDRVBlkioState *s = bs->opaque; + bool needs_mem_regions = + s->needs_mem_regions && !(flags & BDRV_REQ_REGISTERED_BUF); struct iovec *iov = qiov->iov; int iovcnt = qiov->niov; BlkioAIOCB *acb; @@ -266,7 +276,7 @@ static BlockAIOCB *blkio_aio_pwritev(BlockDriverState *bs, int64_t offset, acb = blkio_aiocb_get(bs, cb, opaque); - if (s->needs_mem_regions) { + if (needs_mem_regions) { if (blkio_aiocb_init_mem_region_locked(acb, bytes) < 0) { qemu_aio_unref(&acb->common); return NULL; @@ -352,6 +362,117 @@ static void blkio_io_unplug(BlockDriverState *bs) } } +typedef enum { + BMRR_OK, + BMRR_SKIP, + BMRR_FAIL, +} BlkioMemRegionResult; + +/* + * Produce a struct blkio_mem_region for a given address and size. + * + * This function produces identical results when called multiple times with the + * same arguments. This property is necessary because blkio_unmap_mem_region() + * must receive the same struct blkio_mem_region field values that were passed + * to blkio_map_mem_region(). + */ +static BlkioMemRegionResult +blkio_mem_region_from_host(BlockDriverState *bs, + void *host, size_t size, + struct blkio_mem_region *region, + Error **errp) +{ + BDRVBlkioState *s = bs->opaque; + int fd = -1; + ram_addr_t fd_offset = 0; + + if (((uintptr_t)host | size) % s->mem_region_alignment) { + error_setg(errp, "unaligned buf %p with size %zu", host, size); + return BMRR_FAIL; + } + + /* Attempt to find the fd for the underlying memory */ + if (s->needs_mem_region_fd) { + RAMBlock *ram_block; + RAMBlock *end_block; + ram_addr_t offset; + + /* + * bdrv_register_buf() is called with the BQL held so mr lives at least + * until this function returns. + */ + ram_block = qemu_ram_block_from_host(host, false, &fd_offset); + if (ram_block) { + fd = qemu_ram_get_fd(ram_block); + } + if (fd == -1) { + /* + * Ideally every RAMBlock would have an fd. pc-bios and other + * things don't. Luckily they are usually not I/O buffers and we + * can just ignore them. + */ + return BMRR_SKIP; + } + + /* Make sure the fd covers the entire range */ + end_block = qemu_ram_block_from_host(host + size - 1, false, &offset); + if (ram_block != end_block) { + error_setg(errp, "registered buffer at %p with size %zu extends " + "beyond RAMBlock", host, size); + return BMRR_FAIL; + } + } + + *region = (struct blkio_mem_region){ + .addr = host, + .len = size, + .fd = fd, + .fd_offset = fd_offset, + }; + return BMRR_OK; +} + +static bool blkio_register_buf(BlockDriverState *bs, void *host, size_t size, + Error **errp) +{ + BDRVBlkioState *s = bs->opaque; + struct blkio_mem_region region; + BlkioMemRegionResult region_result; + int ret; + + region_result = blkio_mem_region_from_host(bs, host, size, ®ion, errp); + if (region_result == BMRR_SKIP) { + return true; + } else if (region_result != BMRR_OK) { + return false; + } + + WITH_QEMU_LOCK_GUARD(&s->lock) { + ret = blkio_map_mem_region(s->blkio, ®ion); + } + + if (ret < 0) { + error_setg(errp, "Failed to add blkio mem region %p with size %zu: %s", + host, size, blkio_get_error_msg()); + return false; + } + return true; +} + +static void blkio_unregister_buf(BlockDriverState *bs, void *host, size_t size) +{ + BDRVBlkioState *s = bs->opaque; + struct blkio_mem_region region; + + if (blkio_mem_region_from_host(bs, host, size, ®ion, NULL) != BMRR_OK) { + return; + } + + WITH_QEMU_LOCK_GUARD(&s->lock) { + blkio_unmap_mem_region(s->blkio, ®ion); + } +} + static int blkio_io_uring_open(BlockDriverState *bs, QDict *options, int flags, Error **errp) { @@ -490,6 +611,17 @@ static int blkio_file_open(BlockDriverState *bs, QDict *options, int flags, return ret; } + ret = blkio_get_bool(s->blkio, + "needs-mem-region-fd", + &s->needs_mem_region_fd); + if (ret < 0) { + error_setg_errno(errp, -ret, + "failed to get needs-mem-region-fd: %s", + blkio_get_error_msg()); + blkio_destroy(&s->blkio); + return ret; + } + ret = blkio_get_uint64(s->blkio, "mem-region-alignment", &s->mem_region_alignment); @@ -509,7 +641,7 @@ static int blkio_file_open(BlockDriverState *bs, QDict *options, int flags, return ret; } - bs->supported_write_flags = BDRV_REQ_FUA; + bs->supported_write_flags = BDRV_REQ_FUA | BDRV_REQ_REGISTERED_BUF; bs->supported_zero_flags = BDRV_REQ_FUA | BDRV_REQ_MAY_UNMAP | BDRV_REQ_NO_FALLBACK; @@ -673,6 +805,8 @@ static BlockDriver bdrv_io_uring = { .bdrv_co_pwrite_zeroes = blkio_co_pwrite_zeroes, .bdrv_io_unplug = blkio_io_unplug, .bdrv_refresh_limits = blkio_refresh_limits, + .bdrv_register_buf = blkio_register_buf, + .bdrv_unregister_buf = blkio_unregister_buf, }; static BlockDriver bdrv_virtio_blk_vhost_user = { @@ -713,6 +847,8 @@ static BlockDriver bdrv_virtio_blk_vhost_vdpa = { .bdrv_co_pwrite_zeroes = blkio_co_pwrite_zeroes, .bdrv_io_unplug = blkio_io_unplug, .bdrv_refresh_limits = blkio_refresh_limits, + .bdrv_register_buf = blkio_register_buf, + .bdrv_unregister_buf = blkio_unregister_buf, }; static void bdrv_blkio_init(void) -- 2.37.2