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 !
With regards,
Daniel
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