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 ? > > @@ -209,10 +244,11 @@ void file_start_incoming_migration(FileMigrationArgs > *file_args, Error **errp) > g_autofree char *filename = g_strdup(file_args->filename); > QIOChannelFile *fioc = NULL; > uint64_t offset = file_args->offset; > + int flags = O_RDONLY; > > trace_migration_file_incoming(filename); > > - fioc = qio_channel_file_new_path(filename, O_RDONLY, 0, errp); > + fioc = qio_channel_file_new_path(filename, flags, 0, errp); > if (!fioc) { > return; > } > -- > 2.35.3 > With regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|