Daniel P. Berrangé <berra...@redhat.com> writes: > On Wed, May 08, 2024 at 05:39:53PM -0300, Fabiano Rosas wrote: >> Peter Xu <pet...@redhat.com> writes: >> >> > On Wed, May 08, 2024 at 09:53:48AM +0100, Daniel P. Berrangé wrote: >> >> On Fri, Apr 26, 2024 at 11:20:41AM -0300, Fabiano Rosas wrote: >> >> > Allow multifd to use an fdset when migrating to a file. This is useful >> >> > for the scenario where the management layer wants to have control over >> >> > the migration file. >> >> > >> >> > By receiving the file descriptors directly, QEMU can delegate some >> >> > high level operating system operations to the management layer (such >> >> > as mandatory access control). The management layer might also want to >> >> > add its own headers before the migration stream. >> >> > >> >> > Enable the "file:/dev/fdset/#" syntax for the multifd migration with >> >> > mapped-ram. The requirements for the fdset mechanism are: >> >> > >> >> > On the migration source side: >> >> > >> >> > - the fdset must contain two fds that are not duplicates between >> >> > themselves; >> >> > - if direct-io is to be used, exactly one of the fds must have the >> >> > O_DIRECT flag set; >> >> > - the file must be opened with WRONLY both times. >> >> > >> >> > On the migration destination side: >> >> > >> >> > - the fdset must contain one fd; >> >> > - the file must be opened with RDONLY. >> >> > >> >> > Signed-off-by: Fabiano Rosas <faro...@suse.de> >> >> > --- >> >> > docs/devel/migration/main.rst | 18 ++++++++++++++ >> >> > docs/devel/migration/mapped-ram.rst | 6 ++++- >> >> > migration/file.c | 38 ++++++++++++++++++++++++++++- >> >> > 3 files changed, 60 insertions(+), 2 deletions(-) >> >> > >> >> > diff --git a/docs/devel/migration/main.rst >> >> > b/docs/devel/migration/main.rst >> >> > index 54385a23e5..50f6096470 100644 >> >> > --- a/docs/devel/migration/main.rst >> >> > +++ b/docs/devel/migration/main.rst >> >> > @@ -47,6 +47,24 @@ over any transport. >> >> > QEMU interference. Note that QEMU does not flush cached file >> >> > data/metadata at the end of migration. >> >> > >> >> > + The file migration also supports using a file that has already been >> >> > + opened. A set of file descriptors is passed to QEMU via an "fdset" >> >> > + (see add-fd QMP command documentation). This method allows a >> >> > + management application to have control over the migration file >> >> > + opening operation. There are, however, strict requirements to this >> >> > + interface: >> >> > + >> >> > + On the migration source side: >> >> > + - if the multifd capability is to be used, the fdset must contain >> >> > + two file descriptors that are not duplicates between themselves; >> >> > + - if the direct-io capability is to be used, exactly one of the >> >> > + file descriptors must have the O_DIRECT flag set; >> >> > + - the file must be opened with WRONLY. >> >> > + >> >> > + On the migration destination side: >> >> > + - the fdset must contain one file descriptor; >> >> > + - the file must be opened with RDONLY. >> >> > + >> >> > In addition, support is included for migration using RDMA, which >> >> > transports the page data using ``RDMA``, where the hardware takes care >> >> > of >> >> > transporting the pages, and the load on the CPU is much lower. While >> >> > the >> >> > diff --git a/docs/devel/migration/mapped-ram.rst >> >> > b/docs/devel/migration/mapped-ram.rst >> >> > index fa4cefd9fc..e6505511f0 100644 >> >> > --- a/docs/devel/migration/mapped-ram.rst >> >> > +++ b/docs/devel/migration/mapped-ram.rst >> >> > @@ -16,7 +16,7 @@ location in the file, rather than constantly being >> >> > added to a >> >> > sequential stream. Having the pages at fixed offsets also allows the >> >> > usage of O_DIRECT for save/restore of the migration stream as the >> >> > pages are ensured to be written respecting O_DIRECT alignment >> >> > -restrictions (direct-io support not yet implemented). >> >> > +restrictions. >> >> > >> >> > Usage >> >> > ----- >> >> > @@ -35,6 +35,10 @@ Use a ``file:`` URL for migration: >> >> > Mapped-ram migration is best done non-live, i.e. by stopping the VM on >> >> > the source side before migrating. >> >> > >> >> > +For best performance enable the ``direct-io`` capability as well: >> >> > + >> >> > + ``migrate_set_capability direct-io on`` >> >> > + >> >> > Use-cases >> >> > --------- >> >> > >> >> > diff --git a/migration/file.c b/migration/file.c >> >> > index b9265b14dd..3bc8bc7463 100644 >> >> > --- a/migration/file.c >> >> > +++ b/migration/file.c >> >> > @@ -17,6 +17,7 @@ >> >> > #include "io/channel-file.h" >> >> > #include "io/channel-socket.h" >> >> > #include "io/channel-util.h" >> >> > +#include "monitor/monitor.h" >> >> > #include "options.h" >> >> > #include "trace.h" >> >> > >> >> > @@ -54,10 +55,18 @@ static void file_remove_fdset(void) >> >> > } >> >> > } >> >> > >> >> > +/* >> >> > + * With multifd, due to the behavior of the dup() system call, we need >> >> > + * the fdset to have two non-duplicate fds so we can enable direct IO >> >> > + * in the secondary channels without affecting the main channel. >> >> > + */ >> >> > static bool file_parse_fdset(const char *filename, int64_t *fdset_id, >> >> > Error **errp) >> >> > { >> >> > + FdsetInfoList *fds_info; >> >> > + FdsetFdInfoList *fd_info; >> >> > const char *fdset_id_str; >> >> > + int nfds = 0; >> >> > >> >> > *fdset_id = -1; >> >> > >> >> > @@ -71,6 +80,32 @@ static bool file_parse_fdset(const char *filename, >> >> > int64_t *fdset_id, >> >> > return false; >> >> > } >> >> > >> >> > + if (!migrate_multifd() || !migrate_direct_io()) { >> >> > + return true; >> >> > + } >> >> > + >> >> > + for (fds_info = qmp_query_fdsets(NULL); fds_info; >> >> > + fds_info = fds_info->next) { >> >> > + >> >> > + if (*fdset_id != fds_info->value->fdset_id) { >> >> > + continue; >> >> > + } >> >> > + >> >> > + for (fd_info = fds_info->value->fds; fd_info; fd_info = >> >> > fd_info->next) { >> >> > + if (nfds++ > 2) { >> >> > + break; >> >> > + } >> >> > + } >> >> > + } >> >> > + >> >> > + if (nfds != 2) { >> >> > + error_setg(errp, "Outgoing migration needs two fds in the >> >> > fdset, " >> >> > + "got %d", nfds); >> >> > + qmp_remove_fd(*fdset_id, false, -1, NULL); >> >> > + *fdset_id = -1; >> >> > + return false; >> >> > + } >> >> > + >> >> > return true; >> >> > } >> >> >> >> Related to my thoughts in an earlier patch, where I say that use of fdsets >> >> ought to be transparent to QEMU code, I'm not a fan of having this logic >> >> in migration code. >> >> >> >> IIUC, the migration code will call qio_channel_file_new_path twice, >> >> once with O_DIRECT and once without. This should trigger two calls >> >> into monitor_fdset_dup_fd_add with different flags. If we're matching >> >> flags in that monitor_fdset_dup_fd_add(), then if only 1 FD was >> >> provided, are we not able to report an error there ? >> > >> > Right, this sounds working. >> >> It works, but due to how low-level fdset is, it's difficult to match the >> low level error to anything meaningful we can report to the user. I'll >> have to add an errp to monitor_fdset_dup_fd_add(). Its returns are not >> very useful. >> >> -1 with no errno >> -1 with EACCES (should actually be EBADF) >> -1 with ENOENT >> >> There has been some discusstion around this before actually: >> >> https://lists.gnu.org/archive/html/qemu-devel/2021-08/msg02544.html > > The only caller of monitor_fdset_dup_fd_add is qemu_open_internal > and that has a "Error **errp" parameter. We should rewrite > monitor_fdset_dup_fd_add to also have an "Error **errp" at which > point we can actually report useful, actionable error messages > from it. Errnos be gone !
I can do that, but qemu_open_old() does not pass the error in. Please see if this works for you: -->8-- >From 16e333cc5aeca1fab3f75f79048c0ab0d62d5b08 Mon Sep 17 00:00:00 2001 From: Fabiano Rosas <faro...@suse.de> Date: Fri, 17 May 2024 19:30:39 -0300 Subject: [PATCH] io: Stop using qemu_open_old in channel-file We want to make use of the Error object to report fdset errors from qemu_open_internal() and passing the error pointer to qemu_open_old() would require changing all callers. Move the file channel to the new API instead. Signed-off-by: Fabiano Rosas <faro...@suse.de> --- io/channel-file.c | 8 +++++--- 1 file changed, 5 insertions(+), 3 deletions(-) diff --git a/io/channel-file.c b/io/channel-file.c index 6436cfb6ae..2ea8d08360 100644 --- a/io/channel-file.c +++ b/io/channel-file.c @@ -68,11 +68,13 @@ qio_channel_file_new_path(const char *path, ioc = QIO_CHANNEL_FILE(object_new(TYPE_QIO_CHANNEL_FILE)); - ioc->fd = qemu_open_old(path, flags, mode); + if (flags & O_CREAT) { + ioc->fd = qemu_create(path, flags & ~O_CREAT, mode, errp); + } else { + ioc->fd = qemu_open(path, flags, errp); + } if (ioc->fd < 0) { object_unref(OBJECT(ioc)); - error_setg_errno(errp, errno, - "Unable to open %s", path); return NULL; } -- 2.35.3