Re: [PATCH v6 1/2] io: Add support for MSG_PEEK for socket channel

["Daniel P. Berrangé" <berrange@redhat.com>]

On Thu, Feb 02, 2023 at 01:51:28PM +0100, Juan Quintela wrote:
> Daniel P. Berrangé <berrange@redhat.com> wrote:
> > On Thu, Feb 02, 2023 at 01:22:12PM +0100, Juan Quintela wrote:
> >> "manish.mishra" <manish.mishra@nutanix.com> wrote:
> >> > MSG_PEEK peeks at the channel, The data is treated as unread and
> >> > the next read shall still return this data. This support is
> >> > currently added only for socket class. Extra parameter 'flags'
> >> > is added to io_readv calls to pass extra read flags like MSG_PEEK.
> >> >
> >> > Reviewed-by: Peter Xu <peterx@redhat.com>
> >> > Reviewed-by: Daniel P. Berrange <berrange@redhat.com>
> >> > Suggested-by: Daniel P. Berrange <berrange@redhat.com>
> >> > Signed-off-by: manish.mishra <manish.mishra@nutanix.com>
> >> 
> >> 
> >> This change breaks RDMA migration.
> >> 
> >> FAILED: libcommon.fa.p/migration_rdma.c.o
> >> cc -m64 -mcx16 -Ilibcommon.fa.p -I/usr/include/pixman-1 -I/usr/include/libpng16 -I/usr/include/spice-server -I/usr/include/spice-1 -I/usr/include/cacard -I/usr/include/glib-2.0 -I/usr/lib64/glib-2.0/include -I/usr/include/sysprof-4 -I/usr/include/nss3 -I/usr/include/nspr4 -I/usr/include/PCSC -I/usr/include/p11-kit-1 -I/usr/include/libusb-1.0 -I/usr/include/SDL2 -I/usr/include/libmount -I/usr/include/blkid -I/usr/include/gio-unix-2.0 -I/usr/include/slirp -fdiagnostics-color=auto -Wall -Winvalid-pch -Werror -std=gnu11 -O2 -g -isystem /mnt/code/qemu/full/linux-headers -isystem linux-headers -iquote . -iquote /mnt/code/qemu/full -iquote /mnt/code/qemu/full/include -iquote /mnt/code/qemu/full/tcg/i386 -pthread -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=2 -D_GNU_SOURCE -D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE -fno-strict-aliasing -fno-common -fwrapv -Wundef -Wwrite-strings -Wmissing-prototypes -Wstrict-prototypes -Wredundant-decls -Wold-style-declaration -Wold-style-definition -Wtype-limits -Wformat-security -Wformat-y2k -Winit-self -Wignored-qualifiers -Wempty-body -Wnested-externs -Wendif-labels -Wexpansion-to-defined -Wimplicit-fallthrough=2 -Wmissing-format-attribute -Wno-missing-include-dirs -Wno-shift-negative-value -Wno-psabi -fstack-protector-strong -fPIE -D_REENTRANT -Wno-undef -DSTRUCT_IOVEC_DEFINED -MD -MQ libcommon.fa.p/migration_rdma.c.o -MF libcommon.fa.p/migration_rdma.c.o.d -o libcommon.fa.p/migration_rdma.c.o -c ../../../../mnt/code/qemu/full/migration/rdma.c
> >> ../../../../mnt/code/qemu/full/migration/rdma.c: In function ‘qio_channel_rdma_class_init’:
> >> ../../../../mnt/code/qemu/full/migration/rdma.c:4020:25: error: assignment to ‘ssize_t (*)(QIOChannel *, const struct iovec *, size_t,  int **, size_t *, int,  Error **)’ {aka ‘long int (*)(QIOChannel *, const struct iovec *, long unsigned int,  int **, long unsigned int *, int,  Error **)’} from incompatible pointer type ‘ssize_t (*)(QIOChannel *, const struct iovec *, size_t,  int **, size_t *, Error **)’ {aka ‘long int (*)(QIOChannel *, const struct iovec *, long unsigned int,  int **, long unsigned int *, Error **)’} [-Werror=incompatible-pointer-types]
> >>  4020 |     ioc_klass->io_readv = qio_channel_rdma_readv;
> >>       |                         ^
> >> cc1: all warnings being treated as errors
> >> 
> >> And I don't really know how to fix it, because the problem is that rdma
> >> don't use qio_channel_readv_full() at all.
> >
> > Likely qio_channel_rdma_readv just adds the 'int flags' param added.
> > It doesn't need to actually do anything with the flags as they are
> > checked before
> 
> I can do that.  That would fix the compilation issue.
> 
> But will rdma work?  Because it fakes a qio channel, so what is going to
> implement the MSG_PEEK functionality for it?  It don't end calling
> recv() at all.

It is no problem - the qio_channel_readv method changes in this patch
add:

+    if ((flags & QIO_CHANNEL_READ_FLAG_MSG_PEEK) &&
+        !qio_channel_has_feature(ioc, QIO_CHANNEL_FEATURE_READ_MSG_PEEK)) {
+        error_setg_errno(errp, EINVAL,
+                         "Channel does not support peek read");
+        return -1;
+    }


so it is impossible for qio_channel_rdma_readv to be invoked with
flags having MSG_PEEK set, thus RDMA can ignore the whole concept.

With regards,
Daniel
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