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.
For a real sanity check, we may want to somehow check the two fds returned
from qio_channel_file_new_path() to point to the same file underneath.
What mentioned in the other thread (kcmp with KCMP_FILE) might not work, as
the whole purpose of having two fds is to make sure they have different
struct file to back the fd (and only one of them has O_DIRECT). fstat()
might work in this case over the st_ino field, etc. maybe fstatfs() too but
perhaps that's over cautious. Just a pain to use two fds as a start..
Thanks,
--
Peter Xu
