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

[Daniel P . Berrangé]

On Thu, Feb 02, 2023 at 02:39:05PM +0100, Juan Quintela wrote:
> Daniel P. Berrangé <berrange@redhat.com> wrote:
> > 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.
> 
> And as we require MSG_PEEK to do migration, we have lost RDMA migration
> in the process.
> 
> The following patch on the series use this functionality to read the
> beggining of the streams in the channels.

It guards that usage of MSG_PEEK with

   if (... && qio_channel_has_feature(ioc, QIO_CHANNEL_FEATURE_READ_MSG_PEEK)) {


With regards,
Daniel
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