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 <[email protected]>
> ---
> 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
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