Stefan Hajnoczi <stefa...@redhat.com> writes: > On Wed, Sep 30, 2020 at 07:28:58AM +0200, Markus Armbruster wrote: >> Stefan Hajnoczi <stefa...@redhat.com> writes: >> >> > Use the new QAPI block exports API instead of defining our own QOM >> > objects. >> > >> > This is a large change because the lifecycle of VuBlockDev needs to >> > follow BlockExportDriver. QOM properties are replaced by QAPI options >> > objects. >> > >> > VuBlockDev is renamed VuBlkExport and contains a BlockExport field. >> > Several fields can be dropped since BlockExport already has equivalents. >> > >> > The file names and meson build integration will be adjusted in a future >> > patch. libvhost-user should probably be built as a static library that >> > is linked into QEMU instead of as a .c file that results in duplicate >> > compilation. >> > >> > The new command-line syntax is: >> > >> > $ qemu-storage-daemon \ >> > --blockdev file,node-name=drive0,filename=test.img \ >> > --export >> > vhost-user-blk,node-name=drive0,id=export0,unix-socket=/tmp/vhost-user-blk.sock >> > >> > Note that unix-socket is optional because we may wish to accept chardevs >> > too in the future. >> > >> > Signed-off-by: Stefan Hajnoczi <stefa...@redhat.com> >> > --- >> > v2: >> > * Replace str unix-socket with SocketAddress addr to match NBD and >> > support file descriptor passing >> > * Make addr mandatory [Markus] >> > * Update vhost-user-blk-test.c to use --export syntax >> > --- >> > qapi/block-export.json | 21 +- >> > block/export/vhost-user-blk-server.h | 23 +- >> > block/export/export.c | 8 +- >> > block/export/vhost-user-blk-server.c | 452 +++++++-------------------- >> > tests/qtest/vhost-user-blk-test.c | 2 +- >> > util/vhost-user-server.c | 10 +- >> > block/export/meson.build | 1 + >> > block/meson.build | 1 - >> > 8 files changed, 158 insertions(+), 360 deletions(-) >> > >> > diff --git a/qapi/block-export.json b/qapi/block-export.json >> > index ace0d66e17..2e44625bb1 100644 >> > --- a/qapi/block-export.json >> > +++ b/qapi/block-export.json >> > @@ -84,6 +84,21 @@ >> > 'data': { '*name': 'str', '*description': 'str', >> > '*bitmap': 'str' } } >> > >> > +## >> > +# @BlockExportOptionsVhostUserBlk: >> > +# >> > +# A vhost-user-blk block export. >> > +# >> > +# @addr: The vhost-user socket on which to listen. Both 'unix' and 'fd' >> > +# SocketAddress types are supported. Passed fds must be UNIX domain >> > +# sockets. >> >> "addr.type must be 'unix' or 'fd'" is not visible in introspection. >> Awkward. Practical problem only if other addresses ever become >> available here. Is that possible? > > addr.type=fd itself has the same problem, because it is a file > descriptor without type information. Therefore the QMP client cannot > introspect which types of file descriptors can be passed.
Yes, but if introspection could tell us which which values of addr.type are valid, then a client should figure out the address families, as follows. Any valid value other than 'fd' corresponds to an address family. The set of values valid for addr.type therefore corresponds to a set of address families. The address families in that set are all valid with 'fd', aren't they? > Two ideas: > > 1. Introduce per-address family fd types (SocketAddrFdTcpInet, > SocketAddrFdTcpInet6, SocketAddrFdUnix, etc) to express specific > address families in the QAPI schema. > > Then use per-command unions to express the address families supported > by specific commands. For example, > BlockExportOptionsVhostUserBlkSocketAddr would only allow > SocketAddrUnix and SocketAddrFdUnix. That way new address families > can be supported in the future and introspection reports. Awkward. These types would have to differ structurally, or else they are indistinguishable in introspection. > 2. Use a side-channel (query-features, I think we discussed something > like this a while back) to report features that cannot be > introspected. We implemented this in the form of QAPI feature flags, visible in introspection. You could do something like 'addr': { 'type': 'SocketAddress', 'features': [ 'unix' ] } > I think the added complexity for achieving full introspection is not > worth it. It becomes harder to define new QAPI commands, increases the > chance of errors, and is more tedious to program for clients/servers. Hence my question: is it possible that address families other than unix become available here? When that happens, we have an introspection problem of the sort we common solve with a feature flag. > Accepting any SocketAddr seems reasonable to me since vhost-user > requires an address family that has file descriptor passing. Very few > address families support this feature and we don't expect to add new > ones often. Your answer appears to be "yes in theory, quite unlikely in practice". Correct?