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Thu, 27 Aug 2020 10:32:46 +0000 (UTC) Received: from gondolin (ovpn-113-237.ams2.redhat.com [10.36.113.237]) by smtp.corp.redhat.com (Postfix) with ESMTP id E5D97196F3; Thu, 27 Aug 2020 10:32:42 +0000 (UTC) Date: Thu, 27 Aug 2020 12:32:40 +0200 From: Cornelia Huck To: Tony Krowiak Cc: linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org, freude@linux.ibm.com, borntraeger@de.ibm.com, mjrosato@linux.ibm.com, pasic@linux.ibm.com, alex.williamson@redhat.com, kwankhede@nvidia.com, fiuczy@linux.ibm.com, frankja@linux.ibm.com, david@redhat.com, imbrenda@linux.ibm.com, hca@linux.ibm.com, gor@linux.ibm.com Subject: Re: [PATCH v10 01/16] s390/vfio-ap: add version vfio_ap module Message-ID: <20200827123240.42e0c787.cohuck@redhat.com> In-Reply-To: References: <20200821195616.13554-1-akrowiak@linux.ibm.com> <20200821195616.13554-2-akrowiak@linux.ibm.com> <20200825120432.13a1b444.cohuck@redhat.com> Organization: Red Hat GmbH MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 Sender: kvm-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On Wed, 26 Aug 2020 10:49:47 -0400 Tony Krowiak wrote: > On 8/25/20 6:04 AM, Cornelia Huck wrote: > > On Fri, 21 Aug 2020 15:56:01 -0400 > > Tony Krowiak wrote: > > > >> Let's set a version for the vfio_ap module so that automated regression > >> tests can determine whether dynamic configuration tests can be run or > >> not. > >> > >> Signed-off-by: Tony Krowiak > >> --- > >> drivers/s390/crypto/vfio_ap_drv.c | 2 ++ > >> 1 file changed, 2 insertions(+) > >> > >> diff --git a/drivers/s390/crypto/vfio_ap_drv.c b/drivers/s390/crypto/vfio_ap_drv.c > >> index be2520cc010b..f4ceb380dd61 100644 > >> --- a/drivers/s390/crypto/vfio_ap_drv.c > >> +++ b/drivers/s390/crypto/vfio_ap_drv.c > >> @@ -17,10 +17,12 @@ > >> > >> #define VFIO_AP_ROOT_NAME "vfio_ap" > >> #define VFIO_AP_DEV_NAME "matrix" > >> +#define VFIO_AP_MODULE_VERSION "1.2.0" > >> > >> MODULE_AUTHOR("IBM Corporation"); > >> MODULE_DESCRIPTION("VFIO AP device driver, Copyright IBM Corp. 2018"); > >> MODULE_LICENSE("GPL v2"); > >> +MODULE_VERSION(VFIO_AP_MODULE_VERSION); > >> > >> static struct ap_driver vfio_ap_drv; > >> > > Setting a version manually has some drawbacks: > > - tools wanting to check for capabilities need to keep track which > > versions support which features > > - you need to remember to actually bump the version when adding a new, > > visible feature > > (- selective downstream backports may get into a pickle, but that's > > arguably not your problem) > > > > Is there no way for a tool to figure out whether this is supported? > > E.g., via existence of a sysfs file, or via a known error that will > > occur. If not, it's maybe better to expose known capabilities via a > > generic interface. > > This patch series introduces a new mediated device sysfs attribute, > guest_matrix, so the automated tests could check for the existence > of that interface. The problem I have with that is it will work for > this version of the vfio_ap device driver - which may be all that is > ever needed - but does not account for future enhancements > which may need to be detected by tooling or automated tests. > It seems to me that regardless of how a tool detects whether > a feature is supported or not, it will have to keep track of that > somehow. Which enhancements? If you change the interface in an incompatible way, you have a different problem anyway. If someone trying to use the enhanced version of the interface gets an error on a kernel providing an older version of the interface, that's a reasonable way to discover support. I think "discover device driver capabilities by probing" is less burdensome and error prone than trying to match up capabilities with a version number. If you expose a version number, a tool would still have to probe that version number, and then consult with a list of features per version, which can easily go out of sync. > Can you provide more details about this generic interface of > which you speak? If that is really needed, I'd probably do a driver sysfs attribute that exposes a list of documented capabilities (as integer values, or as a bit.) But since tools can simply check for guest_matrix to find out about support for this feature here, it seems like overkill to me -- unless you have a multitude of features waiting in queue that need to be made discoverable.