From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:57016 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726567AbgKSUEP (ORCPT ); Thu, 19 Nov 2020 15:04:15 -0500 Subject: Re: [RFC PATCH 1/2] vfio-mdev: Wire in a request handler for mdev parent References: <20201117032139.50988-1-farman@linux.ibm.com> <20201117032139.50988-2-farman@linux.ibm.com> <20201119123026.1353cb3c.cohuck@redhat.com> <20201119092754.240847b8@w520.home> From: Eric Farman Message-ID: <27946d84-ae22-6882-67a5-edb5bd782bfa@linux.ibm.com> Date: Thu, 19 Nov 2020 15:04:08 -0500 MIME-Version: 1.0 In-Reply-To: <20201119092754.240847b8@w520.home> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit List-ID: To: Alex Williamson , Cornelia Huck Cc: Kirti Wankhede , Halil Pasic , linux-s390@vger.kernel.org, kvm@vger.kernel.org On 11/19/20 11:27 AM, Alex Williamson wrote: > On Thu, 19 Nov 2020 12:30:26 +0100 > Cornelia Huck wrote: > >> On Tue, 17 Nov 2020 04:21:38 +0100 >> Eric Farman wrote: >> >>> While performing some destructive tests with vfio-ccw, where the >>> paths to a device are forcible removed and thus the device itself >>> is unreachable, it is rather easy to end up in an endless loop in >>> vfio_del_group_dev() due to the lack of a request callback for the >>> associated device. >>> >>> In this example, one MDEV (77c) is used by a guest, while another >>> (77b) is not. The symptom is that the iommu is detached from the >>> mdev for 77b, but not 77c, until that guest is shutdown: >>> >>> [ 238.794867] vfio_ccw 0.0.077b: MDEV: Unregistering >>> [ 238.794996] vfio_mdev 11f2d2bc-4083-431d-a023-eff72715c4f0: Removing from iommu group 2 >>> [ 238.795001] vfio_mdev 11f2d2bc-4083-431d-a023-eff72715c4f0: MDEV: detaching iommu >>> [ 238.795036] vfio_ccw 0.0.077c: MDEV: Unregistering >>> ...silence... >>> >>> Let's wire in the request call back to the mdev device, so that a hot >>> unplug can be (gracefully?) handled by the parent device at the time >>> the device is being removed. >> >> I think it makes a lot of sense to give the vendor driver a way to >> handle requests. >> >>> >>> Signed-off-by: Eric Farman >>> --- >>> drivers/vfio/mdev/vfio_mdev.c | 11 +++++++++++ >>> include/linux/mdev.h | 4 ++++ >>> 2 files changed, 15 insertions(+) >>> >>> diff --git a/drivers/vfio/mdev/vfio_mdev.c b/drivers/vfio/mdev/vfio_mdev.c >>> index 30964a4e0a28..2dd243f73945 100644 >>> --- a/drivers/vfio/mdev/vfio_mdev.c >>> +++ b/drivers/vfio/mdev/vfio_mdev.c >>> @@ -98,6 +98,16 @@ static int vfio_mdev_mmap(void *device_data, struct vm_area_struct *vma) >>> return parent->ops->mmap(mdev, vma); >>> } >>> >>> +static void vfio_mdev_request(void *device_data, unsigned int count) >>> +{ >>> + struct mdev_device *mdev = device_data; >>> + struct mdev_parent *parent = mdev->parent; >>> + >>> + if (unlikely(!parent->ops->request)) >> >> Hm. Do you think that all drivers should implement a ->request() >> callback? > > It's considered optional for bus drivers in vfio-core, obviously > mdev-core could enforce presence of this callback, but then we'd break > existing out of tree drivers. We don't make guarantees to out of tree > drivers, but it feels a little petty. We could instead encourage such > support by printing a warning for drivers that register without a > request callback. Coincidentally, I'd considered adding a dev_warn_once() message in drivers/vfio/vfio.c:vfio_del_group_dev() when vfio_device->ops->request is NULL, and thus we're looping endlessly (and silently). But adding this patch and not patch 2 made things silent again, so I left it out. Putting a warning when the driver registers seems cool. > > Minor nit, I tend to prefer: > > if (callback for thing) > call thing > > Rather than > > if (!callback for thing) > return; > call thing I like it too. I'll set it up that way in v2. > > Thanks, > Alex > >> >>> + return; >>> + parent->ops->request(mdev, count); >>> +} >>> + >>> static const struct vfio_device_ops vfio_mdev_dev_ops = { >>> .name = "vfio-mdev", >>> .open = vfio_mdev_open, >