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Re: [libvirt] [PATCH] qemuOpenVhostNet: Decrease vhostfdSize on open failure



On 05/24/2013 08:50 AM, Michal Privoznik wrote:
> Currently, if there's an error opening /dev/vhost-net (e.g. because
> it doesn't exist) but it's not required we proceed with vhostfd array
> filled with -1 and vhostfdSize unchanged. Later, when constructing
> the qemu command line only non-negative items within vhostfd array
> are taken into account. This means, vhostfdSize may be greater than
> the actual count of non-negative items in vhostfd array. This results
> in improper command line arguments being generated, e.g.:
>
> -netdev tap,fd=21,id=hostnet0,vhost=on,vhostfd=(null)
> ---
>  src/qemu/qemu_command.c | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
>
> diff --git a/src/qemu/qemu_command.c b/src/qemu/qemu_command.c
> index 434f5a7..d969f88 100644
> --- a/src/qemu/qemu_command.c
> +++ b/src/qemu/qemu_command.c
> @@ -486,6 +486,8 @@ qemuOpenVhostNet(virDomainDefPtr def,
>                                         "but is unavailable"));
>                  goto error;
>              }
> +            i--;
> +            (*vhostfdSize)--;

This will still go back through the loop again and try to open another.
I would instead just set vhostfdSize = i (in case there were any
successful opens) and break out of the loop.

And again you'll need to decide what is an error and what gets just a
warning - if someone asks for 3 queues and gets none, that should lead
to continuing without any error or even warning. But if they request 2
and only get one, is that an error, or do we warn and continue? or just
silently continue? (I don't have the answer, I'm just asking the
question :-)

>          }
>      }
>      virDomainAuditNetDevice(def, net, "/dev/vhost-net", *vhostfdSize);
> @@ -6560,12 +6562,10 @@ qemuBuildInterfaceCommandLine(virCommandPtr cmd,
>      }
>  
>      for (i = 0; i < vhostfdSize; i++) {
> -        if (vhostfd[i] >= 0) {
> -            virCommandTransferFD(cmd, vhostfd[i]);
> -            if (virAsprintf(&vhostfdName[i], "%d", vhostfd[i]) < 0) {
> -                virReportOOMError();
> -                goto cleanup;
> -            }
> +        virCommandTransferFD(cmd, vhostfd[i]);
> +        if (virAsprintf(&vhostfdName[i], "%d", vhostfd[i]) < 0) {
> +            virReportOOMError();
> +            goto cleanup;
>          }
>      }
>  


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