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

[Michael S. Tsirkin]

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@xxxxxxxxxx> 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@xxxxxxxxxx>
> > Cc: Igor Mammedov <imammedo@xxxxxxxxxx>
> > Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> > ---
> >  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.

> > +
> >  /* 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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