block/io_uring.c | 13 ++++++++++++- meson.build | 1 + 2 files changed, 13 insertions(+), 1 deletion(-)
Linux recently added a new io_uring(7) optimization API that QEMU
doesn't take advantage of yet. The liburing library that QEMU uses
has added a corresponding new API calling io_uring_register_ring_fd().
When this API is called after creating the ring, the io_uring_submit()
library function passes a flag to the io_uring_enter(2) syscall
allowing it to skip the ring file descriptor fdget()/fdput()
operations. This saves some CPU cycles.
Signed-off-by: Sam Li <faithilikerun@gmail.com>
---
block/io_uring.c | 13 ++++++++++++-
meson.build | 1 +
2 files changed, 13 insertions(+), 1 deletion(-)
diff --git a/block/io_uring.c b/block/io_uring.c
index 0b401512b9..4d691d8373 100644
--- a/block/io_uring.c
+++ b/block/io_uring.c
@@ -17,6 +17,9 @@
#include "qemu/coroutine.h"
#include "qapi/error.h"
#include "trace.h"
+#ifdef CONFIG_LIBURING_REGISTER_RING_FD
+ io_uring_register_ring_fd(&s->ring);
+#endif
/* io_uring ring size */
#define MAX_ENTRIES 128
@@ -434,8 +437,16 @@ LuringState *luring_init(Error **errp)
}
ioq_init(&s->io_q);
- return s;
+ if (io_uring_register_ring_fd(&s->ring) < 0) {
+ /*
+ * Only warn about this error: we will fallback to the non-optimized
+ * io_uring operations.
+ */
+ error_reportf_err(*errp,
+ "failed to register linux io_uring ring file descriptor");
+ }
+ return s;
}
void luring_cleanup(LuringState *s)
diff --git a/meson.build b/meson.build
index 9ebc00f032..927e6ec1a4 100644
--- a/meson.build
+++ b/meson.build
@@ -1733,6 +1733,7 @@ config_host_data.set('CONFIG_LIBNFS', libnfs.found())
config_host_data.set('CONFIG_LIBSSH', libssh.found())
config_host_data.set('CONFIG_LINUX_AIO', libaio.found())
config_host_data.set('CONFIG_LINUX_IO_URING', linux_io_uring.found())
+config_host_data.set('CONFIG_LIBURING_REGISTER_RING_FD', cc.has_function('io_uring_register_ring_fd', prefix: '#include <liburing.h>'))
config_host_data.set('CONFIG_LIBPMEM', libpmem.found())
config_host_data.set('CONFIG_NUMA', numa.found())
config_host_data.set('CONFIG_OPENGL', opengl.found())
--
2.35.3
On Tue, 31 May 2022 at 09:49, Sam Li <faithilikerun@gmail.com> wrote: > > Linux recently added a new io_uring(7) optimization API that QEMU > doesn't take advantage of yet. The liburing library that QEMU uses > has added a corresponding new API calling io_uring_register_ring_fd(). > When this API is called after creating the ring, the io_uring_submit() > library function passes a flag to the io_uring_enter(2) syscall > allowing it to skip the ring file descriptor fdget()/fdput() > operations. This saves some CPU cycles. > > Signed-off-by: Sam Li <faithilikerun@gmail.com> > --- > block/io_uring.c | 13 ++++++++++++- > meson.build | 1 + > 2 files changed, 13 insertions(+), 1 deletion(-) > > diff --git a/block/io_uring.c b/block/io_uring.c > index 0b401512b9..4d691d8373 100644 > --- a/block/io_uring.c > +++ b/block/io_uring.c > @@ -17,6 +17,9 @@ > #include "qemu/coroutine.h" > #include "qapi/error.h" > #include "trace.h" > +#ifdef CONFIG_LIBURING_REGISTER_RING_FD > + io_uring_register_ring_fd(&s->ring); > +#endif When CONFIG_LIBURING_REGISTER_RING_FD is defined the compiler will report an error here since it is not possible to call a function at the top level of a C source file. There is already a io_uring_register_ring_fd() call inside luring_init(), so this one can be removed from the patch. Please move the #ifdef into luring_init() so io_uring_register_ring_fd() is only called when the API is available. > > /* io_uring ring size */ > #define MAX_ENTRIES 128 > @@ -434,8 +437,16 @@ LuringState *luring_init(Error **errp) > } > > ioq_init(&s->io_q); > - return s; > + if (io_uring_register_ring_fd(&s->ring) < 0) { > + /* > + * Only warn about this error: we will fallback to the non-optimized > + * io_uring operations. > + */ > + error_reportf_err(*errp, > + "failed to register linux io_uring ring file descriptor"); Returning a non-NULL LuringState while also setting errp is likely to cause problems. QEMU's error_set() functions have assert(*errp == NULL) so the caller will abort later on if another error occurs after luring_init() set errp. The Error API expects errp to be set zero or one times, not more. Please use warn_report() instead, then errp won't be involved. > + } > > + return s; > } > > void luring_cleanup(LuringState *s) > diff --git a/meson.build b/meson.build > index 9ebc00f032..927e6ec1a4 100644 > --- a/meson.build > +++ b/meson.build > @@ -1733,6 +1733,7 @@ config_host_data.set('CONFIG_LIBNFS', libnfs.found()) > config_host_data.set('CONFIG_LIBSSH', libssh.found()) > config_host_data.set('CONFIG_LINUX_AIO', libaio.found()) > config_host_data.set('CONFIG_LINUX_IO_URING', linux_io_uring.found()) > +config_host_data.set('CONFIG_LIBURING_REGISTER_RING_FD', cc.has_function('io_uring_register_ring_fd', prefix: '#include <liburing.h>')) I checked build/meson-logs/meson-log.txt after compiling to see if io_uring_register_ring_fd() was detected correctly. Detection always fails because meson is compiling the test program without liburing (there is a linker error because the io_uring_register_ring_fd symbol cannot be found). This can be solved by adding a dependency on linux_io_uring: config_host_data.set('CONFIG_LIBURING_REGISTER_RING_FD', cc.has_function('io_uring_register_ring_fd', prefix: '#include <liburing.h>', dependencies: linux_io_uring)) Stefan
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