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blkio: implement BDRV_REQ_REGISTERED_BUF optimization

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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 <stefanha@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Message-id: 20221013185908.1297568-13-stefanha@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
This commit is contained in:
Stefan Hajnoczi 2022-10-13 14:59:07 -04:00
parent 701bff24de
commit c5640b3e2f
1 changed files with 180 additions and 3 deletions
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183
block/blkio.c
View file

@ -11,9 +11,13 @@
#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"
#include "exec/memory.h" /* for ram_block_discard_disable() */
/*
* Keep the QEMU BlockDriver names identical to the libblkio driver names.
@ -73,6 +77,12 @@ 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;
/* Are madvise(MADV_DONTNEED)-style operations unavailable? */
bool may_pin_mem_regions;
} BDRVBlkioState;
/* Called with s->bounce_lock held */
@ -347,7 +357,8 @@ blkio_co_preadv(BlockDriverState *bs, int64_t offset, int64_t bytes,
.coroutine = qemu_coroutine_self(),
};
BDRVBlkioState *s = bs->opaque;
bool use_bounce_buffer = s->needs_mem_regions;
bool use_bounce_buffer =
s->needs_mem_regions && !(flags & BDRV_REQ_REGISTERED_BUF);
BlkioBounceBuf bounce;
struct iovec *iov = qiov->iov;
int iovcnt = qiov->niov;
@ -390,7 +401,8 @@ static int coroutine_fn blkio_co_pwritev(BlockDriverState *bs, int64_t offset,
.coroutine = qemu_coroutine_self(),
};
BDRVBlkioState *s = bs->opaque;
bool use_bounce_buffer = s->needs_mem_regions;
bool use_bounce_buffer =
s->needs_mem_regions && !(flags & BDRV_REQ_REGISTERED_BUF);
BlkioBounceBuf bounce;
struct iovec *iov = qiov->iov;
int iovcnt = qiov->niov;
@ -473,6 +485,130 @@ 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;
/*
* Mapping memory regions conflicts with RAM discard (virtio-mem) when
* there is pinning, so only do it when necessary.
*/
if (!s->needs_mem_regions && s->may_pin_mem_regions) {
return true;
}
region_result = blkio_mem_region_from_host(bs, host, size, &region, errp);
if (region_result == BMRR_SKIP) {
return true;
} else if (region_result != BMRR_OK) {
return false;
}
WITH_QEMU_LOCK_GUARD(&s->blkio_lock) {
ret = blkio_map_mem_region(s->blkio, &region);
}
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;
/* See blkio_register_buf() */
if (!s->needs_mem_regions && s->may_pin_mem_regions) {
return;
}
if (blkio_mem_region_from_host(bs, host, size, &region, NULL) != BMRR_OK) {
return;
}
WITH_QEMU_LOCK_GUARD(&s->blkio_lock) {
blkio_unmap_mem_region(s->blkio, &region);
}
}
static int blkio_io_uring_open(BlockDriverState *bs, QDict *options, int flags,
Error **errp)
{
@ -609,6 +745,17 @@ 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);
@ -620,15 +767,39 @@ static int blkio_file_open(BlockDriverState *bs, QDict *options, int flags,
return ret;
}
ret = blkio_get_bool(s->blkio,
"may-pin-mem-regions",
&s->may_pin_mem_regions);
if (ret < 0) {
/* Be conservative (assume pinning) if the property is not supported */
s->may_pin_mem_regions = s->needs_mem_regions;
}
/*
* Notify if libblkio drivers pin memory and prevent features like
* virtio-mem from working.
*/
if (s->may_pin_mem_regions) {
ret = ram_block_discard_disable(true);
if (ret < 0) {
error_setg_errno(errp, -ret, "ram_block_discard_disable() failed");
blkio_destroy(&s->blkio);
return ret;
}
}
ret = blkio_start(s->blkio);
if (ret < 0) {
error_setg_errno(errp, -ret, "blkio_start failed: %s",
blkio_get_error_msg());
blkio_destroy(&s->blkio);
if (s->may_pin_mem_regions) {
ram_block_discard_disable(false);
}
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;
@ -652,6 +823,10 @@ static void blkio_close(BlockDriverState *bs)
qemu_mutex_destroy(&s->blkio_lock);
blkio_detach_aio_context(bs);
blkio_destroy(&s->blkio);
if (s->may_pin_mem_regions) {
ram_block_discard_disable(false);
}
}
static int64_t blkio_getlength(BlockDriverState *bs)
@ -799,6 +974,8 @@ static void blkio_refresh_limits(BlockDriverState *bs, Error **errp)
.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, \
__VA_ARGS__ \
}