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(dynamic-2a02-3100-a9f3-0900-e8bd-d32d-8176-c56e.310.pool.telefonica.de. [2a02:3100:a9f3:900:e8bd:d32d:8176:c56e]) by smtp.googlemail.com with ESMTPSA id a640c23a62f3a-aa6260e84bfsm366355266b.189.2024.12.07.02.06.14 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sat, 07 Dec 2024 02:06:15 -0800 (PST) Message-ID: <4b9781d5-5cbc-4254-9753-014cf5a8438d@gmail.com> Date: Sat, 7 Dec 2024 11:06:15 +0100 Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 2/3] vfio/mdev: inline needed class_compat functionality To: Greg Kroah-Hartman , Alex Williamson Cc: "Rafael J. Wysocki" , Kirti Wankhede , kvm@vger.kernel.org References: <147a2a3e-8227-4f1b-9ab4-d0b4f261d2a6@gmail.com> <0a14a4df-fbb5-4613-837f-f8025dc73380@gmail.com> <2024120430-boneless-wafer-bf0c@gregkh> <2024120410-promoter-blandness-efa1@gregkh> <20241204123040.7e3483a4.alex.williamson@redhat.com> <9015ce52-e4f3-459c-bd74-b8707cf8fd88@gmail.com> <2024120617-icon-bagel-86b3@gregkh> <20241206093733.1d887dfc.alex.williamson@redhat.com> <2024120721-parasite-thespian-84e0@gregkh> Content-Language: en-US From: Heiner Kallweit Autocrypt: addr=hkallweit1@gmail.com; keydata= xsFNBF/0ZFUBEAC0eZyktSE7ZNO1SFXL6cQ4i4g6Ah3mOUIXSB4pCY5kQ6OLKHh0FlOD5/5/ sY7IoIouzOjyFdFPnz4Bl3927ClT567hUJJ+SNaFEiJ9vadI6vZm2gcY4ExdIevYHWe1msJF MVE4yNwdS+UsPeCF/6CQQTzHc+n7DomE7fjJD5J1hOJjqz2XWe71fTvYXzxCFLwXXbBiqDC9 dNqOe5odPsa4TsWZ09T33g5n2nzTJs4Zw8fCy8rLqix/raVsqr8fw5qM66MVtdmEljFaJ9N8 /W56qGCp+H8Igk/F7CjlbWXiOlKHA25mPTmbVp7VlFsvsmMokr/imQr+0nXtmvYVaKEUwY2g 86IU6RAOuA8E0J5bD/BeyZdMyVEtX1kT404UJZekFytJZrDZetwxM/cAH+1fMx4z751WJmxQ J7mIXSPuDfeJhRDt9sGM6aRVfXbZt+wBogxyXepmnlv9K4A13z9DVLdKLrYUiu9/5QEl6fgI kPaXlAZmJsQfoKbmPqCHVRYj1lpQtDM/2/BO6gHASflWUHzwmBVZbS/XRs64uJO8CB3+V3fa cIivllReueGCMsHh6/8wgPAyopXOWOxbLsZ291fmZqIR0L5Y6b2HvdFN1Xhc+YrQ8TKK+Z4R mJRDh0wNQ8Gm89g92/YkHji4jIWlp2fwzCcx5+lZCQ1XdqAiHQARAQABzSZIZWluZXIgS2Fs bHdlaXQgPGhrYWxsd2VpdDFAZ21haWwuY29tPsLBjgQTAQgAOBYhBGxfqY/yOyXjyjJehXLe ig9U8DoMBQJf9GRVAhsDBQsJCAcCBhUKCQgLAgQWAgMBAh4BAheAAAoJEHLeig9U8DoMSycQ AJbfg8HZEK0ljV4M8nvdaiNixWAufrcZ+SD8zhbxl8GispK4F3Yo+20Y3UoZ7FcIidJWUUJL axAOkpI/70YNhlqAPMsuudlAieeYZKjIv1WV5ucNZ3VJ7dC+dlVqQdAr1iD869FZXvy91KhJ wYulyCf+s4T9YgmLC6jLMBZghKIf1uhSd0NzjyCqYWbk2ZxByZHgunEShOhHPHswu3Am0ftt ePaYIHgZs+Vzwfjs8I7EuW/5/f5G9w1vibXxtGY/GXwgGGHRDjFM7RSprGOv4F5eMGh+NFUJ TU9N96PQYMwXVxnQfRXl8O6ffSVmFx4H9rovxWPKobLmqQL0WKLLVvA/aOHCcMKgfyKRcLah 57vGC50Ga8oT2K1g0AhKGkyJo7lGXkMu5yEs0m9O+btqAB261/E3DRxfI1P/tvDZpLJKtq35 dXsj6sjvhgX7VxXhY1wE54uqLLHY3UZQlmH3QF5t80MS7/KhxB1pO1Cpcmkt9hgyzH8+5org +9wWxGUtJWNP7CppY+qvv3SZtKJMKsxqk5coBGwNkMms56z4qfJm2PUtJQGjA65XWdzQACib 2iaDQoBqGZfXRdPT0tC1H5kUJuOX4ll1hI/HBMEFCcO8++Bl2wcrUsAxLzGvhINVJX2DAQaF aNetToazkCnzubKfBOyiTqFJ0b63c5dqziAgzsFNBF/0ZFUBEADF8UEZmKDl1w/UxvjeyAeX kghYkY3bkK6gcIYXdLRfJw12GbvMioSguvVzASVHG8h7NbNjk1yur6AONfbUpXKSNZ0skV8V fG+ppbaY+zQofsSMoj5gP0amwbwvPzVqZCYJai81VobefTX2MZM2Mg/ThBVtGyzV3NeCpnBa 8AX3s9rrX2XUoCibYotbbxx9afZYUFyflOc7kEpc9uJXIdaxS2Z6MnYLHsyVjiU6tzKCiVOU KJevqvzPXJmy0xaOVf7mhFSNQyJTrZpLa+tvB1DQRS08CqYtIMxRrVtC0t0LFeQGly6bOngr ircurWJiJKbSXVstLHgWYiq3/GmCSx/82ObeLO3PftklpRj8d+kFbrvrqBgjWtMH4WtK5uN5 1WJ71hWJfNchKRlaJ3GWy8KolCAoGsQMovn/ZEXxrGs1ndafu47yXOpuDAozoHTBGvuSXSZo ythk/0EAuz5IkwkhYBT1MGIAvNSn9ivE5aRnBazugy0rTRkVggHvt3/7flFHlGVGpBHxFUwb /a4UjJBPtIwa4tWR8B1Ma36S8Jk456k2n1id7M0LQ+eqstmp6Y+UB+pt9NX6t0Slw1NCdYTW gJezWTVKF7pmTdXszXGxlc9kTrVUz04PqPjnYbv5UWuDd2eyzGjrrFOsJEi8OK2d2j4FfF++ AzOMdW09JVqejQARAQABwsF2BBgBCAAgFiEEbF+pj/I7JePKMl6Fct6KD1TwOgwFAl/0ZFUC GwwACgkQct6KD1TwOgxUfg//eAoYc0Vm4NrxymfcY30UjHVD0LgSvU8kUmXxil3qhFPS7KA+ y7tgcKLHOkZkXMX5MLFcS9+SmrAjSBBV8omKoHNo+kfFx/dUAtz0lot8wNGmWb+NcHeKM1eb nwUMOEa1uDdfZeKef/U/2uHBceY7Gc6zPZPWgXghEyQMTH2UhLgeam8yglyO+A6RXCh+s6ak Wje7Vo1wGK4eYxp6pwMPJXLMsI0ii/2k3YPEJPv+yJf90MbYyQSbkTwZhrsokjQEaIfjrIk3 rQRjTve/J62WIO28IbY/mENuGgWehRlTAbhC4BLTZ5uYS0YMQCR7v9UGMWdNWXFyrOB6PjSu Trn9MsPoUc8qI72mVpxEXQDLlrd2ijEWm7Nrf52YMD7hL6rXXuis7R6zY8WnnBhW0uCfhajx q+KuARXC0sDLztcjaS3ayXonpoCPZep2Bd5xqE4Ln8/COCslP7E92W1uf1EcdXXIrx1acg21 H/0Z53okMykVs3a8tECPHIxnre2UxKdTbCEkjkR4V6JyplTS47oWMw3zyI7zkaadfzVFBxk2 lo/Tny+FX1Azea3Ce7oOnRUEZtWSsUidtIjmL8YUQFZYm+JUIgfRmSpMFq8JP4VH43GXpB/S OCrl+/xujzvoUBFV/cHKjEQYBxo+MaiQa1U54ykM2W4DnHb1UiEf5xDkFd4= In-Reply-To: <2024120721-parasite-thespian-84e0@gregkh> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 07.12.2024 09:38, Greg Kroah-Hartman wrote: > On Fri, Dec 06, 2024 at 09:37:33AM -0700, Alex Williamson wrote: >> On Fri, 6 Dec 2024 08:42:02 +0100 >> Greg Kroah-Hartman wrote: >> >>> On Fri, Dec 06, 2024 at 08:35:47AM +0100, Heiner Kallweit wrote: >>>> On 04.12.2024 20:30, Alex Williamson wrote: >>>>> On Wed, 4 Dec 2024 19:16:03 +0100 >>>>> Greg Kroah-Hartman wrote: >>>>> >>>>>> On Wed, Dec 04, 2024 at 06:01:36PM +0100, Heiner Kallweit wrote: >>>>>>> On 04.12.2024 10:32, Greg Kroah-Hartman wrote: >>>>>>>> On Tue, Dec 03, 2024 at 09:11:47PM +0100, Heiner Kallweit wrote: >>>>>>>>> vfio/mdev is the last user of class_compat, and it doesn't use it for >>>>>>>>> the intended purpose. See kdoc of class_compat_register(): >>>>>>>>> Compatibility class are meant as a temporary user-space compatibility >>>>>>>>> workaround when converting a family of class devices to a bus devices. >>>>>>>> >>>>>>>> True, so waht is mdev doing here? >>>>>>>> >>>>>>>>> In addition it uses only a part of the class_compat functionality. >>>>>>>>> So inline the needed functionality, and afterwards all class_compat >>>>>>>>> code can be removed. >>>>>>>>> >>>>>>>>> No functional change intended. Compile-tested only. >>>>>>>>> >>>>>>>>> Signed-off-by: Heiner Kallweit >>>>>>>>> --- >>>>>>>>> drivers/vfio/mdev/mdev_core.c | 12 ++++++------ >>>>>>>>> 1 file changed, 6 insertions(+), 6 deletions(-) >>>>>>>>> >>>>>>>>> diff --git a/drivers/vfio/mdev/mdev_core.c b/drivers/vfio/mdev/mdev_core.c >>>>>>>>> index ed4737de4..a22c49804 100644 >>>>>>>>> --- a/drivers/vfio/mdev/mdev_core.c >>>>>>>>> +++ b/drivers/vfio/mdev/mdev_core.c >>>>>>>>> @@ -18,7 +18,7 @@ >>>>>>>>> #define DRIVER_AUTHOR "NVIDIA Corporation" >>>>>>>>> #define DRIVER_DESC "Mediated device Core Driver" >>>>>>>>> >>>>>>>>> -static struct class_compat *mdev_bus_compat_class; >>>>>>>>> +static struct kobject *mdev_bus_kobj; >>>>>>>> >>>>>>>> >>>>>>>> >>>>>>>>> >>>>>>>>> static LIST_HEAD(mdev_list); >>>>>>>>> static DEFINE_MUTEX(mdev_list_lock); >>>>>>>>> @@ -76,7 +76,7 @@ int mdev_register_parent(struct mdev_parent *parent, struct device *dev, >>>>>>>>> if (ret) >>>>>>>>> return ret; >>>>>>>>> >>>>>>>>> - ret = class_compat_create_link(mdev_bus_compat_class, dev, NULL); >>>>>>>>> + ret = sysfs_create_link(mdev_bus_kobj, &dev->kobj, dev_name(dev)); >>>>>>>> >>>>>>>> This feels really wrong, why create a link to a random kobject? Who is >>>>>>>> using this kobject link? >>>>>>>> >>>>>>>>> if (ret) >>>>>>>>> dev_warn(dev, "Failed to create compatibility class link\n"); >>>>>>>>> >>>>>>>>> @@ -98,7 +98,7 @@ void mdev_unregister_parent(struct mdev_parent *parent) >>>>>>>>> dev_info(parent->dev, "MDEV: Unregistering\n"); >>>>>>>>> >>>>>>>>> down_write(&parent->unreg_sem); >>>>>>>>> - class_compat_remove_link(mdev_bus_compat_class, parent->dev, NULL); >>>>>>>>> + sysfs_remove_link(mdev_bus_kobj, dev_name(parent->dev)); >>>>>>>>> device_for_each_child(parent->dev, NULL, mdev_device_remove_cb); >>>>>>>>> parent_remove_sysfs_files(parent); >>>>>>>>> up_write(&parent->unreg_sem); >>>>>>>>> @@ -251,8 +251,8 @@ static int __init mdev_init(void) >>>>>>>>> if (ret) >>>>>>>>> return ret; >>>>>>>>> >>>>>>>>> - mdev_bus_compat_class = class_compat_register("mdev_bus"); >>>>>>>>> - if (!mdev_bus_compat_class) { >>>>>>>>> + mdev_bus_kobj = class_pseudo_register("mdev_bus"); >>>>>>>> >>>>>>>> But this isn't a class, so let's not fake it please. Let's fix this >>>>>>>> properly, odds are all of this code can just be removed entirely, right? >>>>>>>> >>>>>>> >>>>>>> After I removed class_compat from i2c core, I asked Alex basically the >>>>>>> same thing: whether class_compat support can be removed from vfio/mdev too. >>>>>>> >>>>>>> His reply: >>>>>>> I'm afraid we have active userspace tools dependent on >>>>>>> /sys/class/mdev_bus currently, libvirt for one. We link mdev parent >>>>>>> devices here and I believe it's the only way for userspace to find >>>>>>> those parent devices registered for creating mdev devices. If there's a >>>>>>> desire to remove class_compat, we might need to add some mdev >>>>>>> infrastructure to register the class ourselves to maintain the parent >>>>>>> links. >>>>>>> >>>>>>> >>>>>>> It's my understanding that /sys/class/mdev_bus has nothing in common >>>>>>> with an actual class, it's just a container for devices which at least >>>>>>> partially belong to other classes. And there's user space tools depending >>>>>>> on this structure. >>>>>> >>>>>> That's odd, when this was added, why was it added this way? The >>>>>> class_compat stuff is for when classes move around, yet this was always >>>>>> done in this way? >>>>>> >>>>>> And what tools use this symlink today that can't be updated? >>>>> >>>>> It's been this way for 8 years, so it's hard to remember exact >>>>> motivation for using this mechanism, whether we just didn't look hard >>>>> enough at the class_compat or it didn't pass by the right set of eyes. >>>>> Yes, it's always been this way for the mdev_bus class. >>>>> >>>>> The intention here is much like other classes, that we have a node in >>>>> sysfs where we can find devices that provide a common feature, in this >>>>> case providing support for creating and managing vfio mediated devices >>>>> (mdevs). The perhaps unique part of this use case is that these devices >>>>> aren't strictly mdev providers, they might also belong to another class >>>>> and the mdev aspect of their behavior might be dynamically added after >>>>> the device itself is added. >>>>> >>>>> I've done some testing with this series and it does indeed seem to >>>>> maintain compatibility with existing userspace tools, mdevctl and >>>>> libvirt. We can update these tools, but then we get into the breaking >>>> >>>> Greg, is this testing, done by Alex, sufficient for you to take the series? >>> >>> Were devices actually removed from the system and all worked well? >> >> Creating and removing virtual mdev devices as well as unloading and >> re-loading modules for the parent device providing the mdev support. >> >>>>> userspace and deprecation period questions, where we may further delay >>>>> removal of class_compat. Also if we were to re-implement this, is there >>>>> a better mechanism than proposed here within the class hierarchy, or >>>>> would you recommend a non-class approach? Thanks, >>>>> >>>> >>>> You have /sys/bus/mdev. Couldn't we create a directory here which holds >>>> the links to the devices in question? >>> >>> Links to devices is not what class links are for, so why is this in >>> /sys/class/ at all? >> >> Sorry, I'm confused. I look in /sys/class/block and /sys/class/net and >> I only see links to devices. > > Yes, they are linking to "class devices", i.e. things controlled by that > class, NOT just a driver bound to a device on a bus. > >> /sys/class/mdev_bus has links to devices >> that have registered as supporting the mdev interface for creating >> devices in /sys/bus/mdev. > > And that's the issue here, drivers binding to a device on a bus are NOT > class devices, they are bus devices (i.e. on the bus.) > > This used to be much "clearer" when we had "struct class_device" but we > unified that a long time ago and now only have "struct device". > > In short, a bus device is the thing that is on a "bus" that a driver > binds to (i.e. controls the hardware). A class device is the thing that > a user talks to in a standardized way (input, block, tty, network, etc.) > that is INDEPENDENT of the hardware bus the device happens to be > attached to. > >> We could link these devices somewhere else, >> but there are existing projects, userspace scripts, and documentation >> that references and relies on this layout. Whatever we decide it >> should have been 8 years ago is going to need yet another compatibility >> interface/link to avoid breaking userspace. > > What you did here is say "mdev is both a standard interface to talk to > userspace AND a standard bus", when really you should have made a mdev > class if you really wanted one. Why not just do that now instead? > Nothing should be preventing that and then your bus code can be the same > too. > >>>> Then user space would simply have to switch from /sys/class/mdev_bus >>>> to /sys/bus/mdev/. >>> >>> I think you are confusing what /sys/class/ is for here, if you have >>> devices on a "bus" then they need to be in /sys/bus/ class has nothing >>> to do with that. >>> >>> So can we just drop the /sys/class/ mistake entirely? >> >> Not without breaking userspace. /sys/bus/mdev is used for enumerating >> the virtual mdev devics that are created by devices supporting the mdev >> interface, where the latter are enumerated in /sys/class/mdev_bus. > > Great, how about creating a mdev_bus "struct class" then and doing this > properly? That would be the correct solution overall, not this > overloading of a symlink that causes confusion. > Issue with this approach is that these "mdev parent" devices are partially class devices belonging to other classes. See for example mtty_dev_init(), there the device passed to us belongs to class mtty. Here in mdev two types of devices are handled: 1. The "mdev parent" devices, which are linked to /sys/class/mdev_bus. These devices can be class devices of other classes. 2. The mdev devices, these are normal bus devices residing under /sys/bus/mdev. > thanks, > > greg k-h