Re: [PATCH RFC] vhost: add ioctl to query nregions upper limit

[Igor Mammedov <imammedo@redhat.com>]

On Wed, 24 Jun 2015 16:17:46 +0200
"Michael S. Tsirkin" <mst@redhat.com> wrote:

> On Wed, Jun 24, 2015 at 04:07:27PM +0200, Igor Mammedov wrote:
> > On Wed, 24 Jun 2015 15:49:27 +0200
> > "Michael S. Tsirkin" <mst@redhat.com> wrote:
> > 
> > > Userspace currently simply tries to give vhost as many regions
> > > as it happens to have, but you only have the mem table
> > > when you have initialized a large part of VM, so graceful
> > > failure is very hard to support.
> > > 
> > > The result is that userspace tends to fail catastrophically.
> > > 
> > > Instead, add a new ioctl so userspace can find out how much kernel
> > > supports, up front. This returns a positive value that we commit to.
> > > 
> > > Also, document our contract with legacy userspace: when running on an
> > > old kernel, you get -1 and you can assume at least 64 slots.  Since 0
> > > value's left unused, let's make that mean that the current userspace
> > > behaviour (trial and error) is required, just in case we want it back.
> > > 
> > > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> > > Cc: Igor Mammedov <imammedo@redhat.com>
> > > Cc: Paolo Bonzini <pbonzini@redhat.com>
> > > ---
> > >  include/uapi/linux/vhost.h | 17 ++++++++++++++++-
> > >  drivers/vhost/vhost.c      |  5 +++++
> > >  2 files changed, 21 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/include/uapi/linux/vhost.h b/include/uapi/linux/vhost.h
> > > index ab373191..f71fa6d 100644
> > > --- a/include/uapi/linux/vhost.h
> > > +++ b/include/uapi/linux/vhost.h
> > > @@ -80,7 +80,7 @@ struct vhost_memory {
> > >   * Allows subsequent call to VHOST_OWNER_SET to succeed. */
> > >  #define VHOST_RESET_OWNER _IO(VHOST_VIRTIO, 0x02)
> > >  
> > > -/* Set up/modify memory layout */
> > > +/* Set up/modify memory layout: see also VHOST_GET_MEM_MAX_NREGIONS below. */
> > >  #define VHOST_SET_MEM_TABLE	_IOW(VHOST_VIRTIO, 0x03, struct vhost_memory)
> > >  
> > >  /* Write logging setup. */
> > > @@ -127,6 +127,21 @@ struct vhost_memory {
> > >  /* Set eventfd to signal an error */
> > >  #define VHOST_SET_VRING_ERR _IOW(VHOST_VIRTIO, 0x22, struct vhost_vring_file)
> > >  
> > > +/* Query upper limit on nregions in VHOST_SET_MEM_TABLE arguments.
> > > + * Returns:
> > > + * 	0 < value <= MAX_INT - gives the upper limit, higher values will fail
> > > + * 	0 - there's no static limit: try and see if it works
> > > + * 	-1 - on failure
> > > + */
> > > +#define VHOST_GET_MEM_MAX_NREGIONS   _IO(VHOST_VIRTIO, 0x23)
> > > +
> > > +/* Returned by VHOST_GET_MEM_MAX_NREGIONS to mean there's no static limit:
> > > + * try and it'll work if you are lucky. */
> > > +#define VHOST_MEM_MAX_NREGIONS_NONE 0
> > is it needed? we always have a limit,
> > or don't have IOCTL => -1 => old try and see way
> > 
> > > +/* We support at least as many nregions in VHOST_SET_MEM_TABLE:
> > > + * for use on legacy kernels without VHOST_GET_MEM_MAX_NREGIONS support. */
> > > +#define VHOST_MEM_MAX_NREGIONS_DEFAULT 64
> > ^^^ not used below,
> > if it's for legacy then perhaps s/DEFAULT/LEGACY/ 
> 
> The assumption was that userspace detecting old kernels will just use 64,
> this means we do want a flag to get the old way.
> 
> OTOH if you won't think it's useful, let me know.
this header will be synced into QEMU's tree so that we could use this define there,
isn't it? IMHO then _LEGACY is more exact description of macro.

As for 0 return value, -1 is just fine for detecting old kernels (i.e. try and see if it works), so 0 looks unnecessary but it doesn't in any way hurt either.
For me limit or -1 is enough to try fix userspace.

> 
> > > +
> > >  /* VHOST_NET specific defines */
> > >  
> > >  /* Attach virtio net ring to a raw socket, or tap device.
> > > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> > > index 9e8e004..3b68f9d 100644
> > > --- a/drivers/vhost/vhost.c
> > > +++ b/drivers/vhost/vhost.c
> > > @@ -917,6 +917,11 @@ long vhost_dev_ioctl(struct vhost_dev *d, unsigned int ioctl, void __user *argp)
> > >  	long r;
> > >  	int i, fd;
> > >  
> > > +	if (ioctl == VHOST_GET_MEM_MAX_NREGIONS) {
> > > +		r = VHOST_MEMORY_MAX_NREGIONS;
> > > +		goto done;
> > > +	}
> > > +
> > >  	/* If you are not the owner, you can become one */
> > >  	if (ioctl == VHOST_SET_OWNER) {
> > >  		r = vhost_dev_set_owner(d);
> --
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