From mboxrd@z Thu Jan 1 00:00:00 1970 From: Igor Mammedov Subject: Re: [PATCH RFC] vhost: add ioctl to query nregions upper limit Date: Wed, 24 Jun 2015 16:07:27 +0200 Message-ID: <20150624160727.2e0f542f@nial.brq.redhat.com> References: <1435153753-2686-1-git-send-email-mst@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <1435153753-2686-1-git-send-email-mst-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> Sender: linux-api-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: "Michael S. Tsirkin" Cc: linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Paolo Bonzini , kvm-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, virtualization-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org, netdev-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-api-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-Id: linux-api@vger.kernel.org On Wed, 24 Jun 2015 15:49:27 +0200 "Michael S. Tsirkin" 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 > Cc: Igor Mammedov > Cc: Paolo Bonzini > --- > 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/ > + > /* 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);