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Tue, 13 Jun 2023 07:42:20 -0700 (PDT) X-Google-Smtp-Source: ACHHUZ4rchrvAq7uL6qdxBGPgYc5Il7VncBPd83zFXMFGzPeWvh3heO7+g3e0YZao/Y4oWkkn0QpGA== X-Received: by 2002:a6b:7b45:0:b0:77a:ec0c:5907 with SMTP id m5-20020a6b7b45000000b0077aec0c5907mr8777918iop.13.1686667340197; Tue, 13 Jun 2023 07:42:20 -0700 (PDT) Received: from redhat.com ([38.15.36.239]) by smtp.gmail.com with ESMTPSA id e5-20020a02caa5000000b0040908cbbc5asm3525944jap.68.2023.06.13.07.42.19 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 13 Jun 2023 07:42:19 -0700 (PDT) Date: Tue, 13 Jun 2023 08:42:18 -0600 From: Alex Williamson To: "Liu, Yi L" Cc: "jgg@nvidia.com" , "Tian, Kevin" , "joro@8bytes.org" , "robin.murphy@arm.com" , "cohuck@redhat.com" , "eric.auger@redhat.com" , "nicolinc@nvidia.com" , "kvm@vger.kernel.org" , "mjrosato@linux.ibm.com" , "chao.p.peng@linux.intel.com" , "yi.y.sun@linux.intel.com" , "peterx@redhat.com" , "jasowang@redhat.com" , "shameerali.kolothum.thodi@huawei.com" , "lulu@redhat.com" , "suravee.suthikulpanit@amd.com" , "intel-gvt-dev@lists.freedesktop.org" , "intel-gfx@lists.freedesktop.org" , "linux-s390@vger.kernel.org" , "Hao, Xudong" , "Zhao, Yan Y" , "Xu, Terrence" , "Jiang, Yanting" , "Duan, Zhenzhong" , "clegoate@redhat.com" Subject: Re: [PATCH v12 07/24] vfio: Block device access via device fd until device is opened Message-ID: <20230613084218.169f1c4c.alex.williamson@redhat.com> In-Reply-To: References: <20230602121653.80017-1-yi.l.liu@intel.com> <20230602121653.80017-8-yi.l.liu@intel.com> <20230612155210.5fd3579f.alex.williamson@redhat.com> <20230613081647.740f5217.alex.williamson@redhat.com> X-Mailer: Claws Mail 4.1.1 (GTK 3.24.35; x86_64-redhat-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-s390@vger.kernel.org On Tue, 13 Jun 2023 14:36:14 +0000 "Liu, Yi L" wrote: > > From: Alex Williamson > > Sent: Tuesday, June 13, 2023 10:17 PM > > > > On Tue, 13 Jun 2023 05:46:32 +0000 > > "Liu, Yi L" wrote: > > > > > > From: Alex Williamson > > > > Sent: Tuesday, June 13, 2023 5:52 AM > > > > > > > > On Fri, 2 Jun 2023 05:16:36 -0700 > > > > Yi Liu wrote: > > > > > > > > > Allow the vfio_device file to be in a state where the device FD is > > > > > opened but the device cannot be used by userspace (i.e. its .open_device() > > > > > hasn't been called). This inbetween state is not used when the device > > > > > FD is spawned from the group FD, however when we create the device FD > > > > > directly by opening a cdev it will be opened in the blocked state. > > > > > > > > > > The reason for the inbetween state is that userspace only gets a FD but > > > > > doesn't gain access permission until binding the FD to an iommufd. So in > > > > > the blocked state, only the bind operation is allowed. Completing bind > > > > > will allow user to further access the device. > > > > > > > > > > This is implemented by adding a flag in struct vfio_device_file to mark > > > > > the blocked state and using a simple smp_load_acquire() to obtain the > > > > > flag value and serialize all the device setup with the thread accessing > > > > > this device. > > > > > > > > > > Following this lockless scheme, it can safely handle the device FD > > > > > unbound->bound but it cannot handle bound->unbound. To allow this we'd > > > > > need to add a lock on all the vfio ioctls which seems costly. So once > > > > > device FD is bound, it remains bound until the FD is closed. > > > > > > > > > > Suggested-by: Jason Gunthorpe > > > > > Reviewed-by: Kevin Tian > > > > > Reviewed-by: Jason Gunthorpe > > > > > Reviewed-by: Eric Auger > > > > > Tested-by: Terrence Xu > > > > > Tested-by: Nicolin Chen > > > > > Tested-by: Matthew Rosato > > > > > Tested-by: Yanting Jiang > > > > > Tested-by: Shameer Kolothum > > > > > Signed-off-by: Yi Liu > > > > > --- > > > > > drivers/vfio/group.c | 11 ++++++++++- > > > > > drivers/vfio/vfio.h | 1 + > > > > > drivers/vfio/vfio_main.c | 16 ++++++++++++++++ > > > > > 3 files changed, 27 insertions(+), 1 deletion(-) > > > > > > > > > > diff --git a/drivers/vfio/group.c b/drivers/vfio/group.c > > > > > index caf53716ddb2..088dd34c8931 100644 > > > > > --- a/drivers/vfio/group.c > > > > > +++ b/drivers/vfio/group.c > > > > > @@ -194,9 +194,18 @@ static int vfio_df_group_open(struct vfio_device_file *df) > > > > > df->iommufd = device->group->iommufd; > > > > > > > > > > ret = vfio_df_open(df); > > > > > - if (ret) > > > > > + if (ret) { > > > > > df->iommufd = NULL; > > > > > + goto out_put_kvm; > > > > > + } > > > > > + > > > > > + /* > > > > > + * Paired with smp_load_acquire() in vfio_device_fops::ioctl/ > > > > > + * read/write/mmap and vfio_file_has_device_access() > > > > > + */ > > > > > + smp_store_release(&df->access_granted, true); > > > > > > > > > > +out_put_kvm: > > > > > if (device->open_count == 0) > > > > > vfio_device_put_kvm(device); > > > > > > > > > > diff --git a/drivers/vfio/vfio.h b/drivers/vfio/vfio.h > > > > > index f9eb52eb9ed7..fdf2fc73f880 100644 > > > > > --- a/drivers/vfio/vfio.h > > > > > +++ b/drivers/vfio/vfio.h > > > > > @@ -18,6 +18,7 @@ struct vfio_container; > > > > > > > > > > struct vfio_device_file { > > > > > struct vfio_device *device; > > > > > + bool access_granted; > > > > > > > > Should we make this a more strongly defined data type and later move > > > > devid (u32) here to partially fill the hole created? > > > > > > Before your question, let me describe how I place the fields > > > of this structure to see if it is common practice. The first two > > > fields are static, so they are in the beginning. The access_granted > > > is lockless and other fields are protected by locks. So I tried to > > > put the lock and the fields it protects closely. So this is why I put > > > devid behind iommufd as both are protected by the same lock. > > > > I think the primary considerations are locality and compactness. Hot > > paths data should be within the first cache line of the structure, > > related data should share a cache line, and we should use the space > > efficiently. What you describe seems largely an aesthetic concern, > > which was not evident to me by the segmentation alone. > > Sure. > > > > > > struct vfio_device_file { > > > struct vfio_device *device; > > > struct vfio_group *group; > > > > > > bool access_granted; > > > spinlock_t kvm_ref_lock; /* protect kvm field */ > > > struct kvm *kvm; > > > struct iommufd_ctx *iommufd; /* protected by struct vfio_device_set::lock */ > > > u32 devid; /* only valid when iommufd is valid */ > > > }; > > > > > > > > > > > I think this is being placed towards the front of the data structure > > > > for cache line locality given this is a hot path for file operations. > > > > But bool types have an implementation dependent size, making them > > > > difficult to pack. Also there will be a tendency to want to make this > > > > a bit field, which is probably not compatible with the smp lockless > > > > operations being used here. We might get in front of these issues if > > > > we just define it as a u8 now. Thanks, > > > > > > Not quite get why bit field is going to be incompatible with smp > > > lockless operations. Could you elaborate a bit? And should I define > > > the access_granted as u8 or "u8:1"? > > > > Perhaps FUD on my part, but load-acquire type operations have specific > > semantics and it's not clear to me that they interest with compiler > > generated bit operations. Thanks, > > I see. How about below? > > struct vfio_device_file { > struct vfio_device *device; > struct vfio_group *group; > u8 access_granted; > u32 devid; /* only valid when iommufd is valid */ > spinlock_t kvm_ref_lock; /* protect kvm field */ > struct kvm *kvm; > struct iommufd_ctx *iommufd; /* protected by struct vfio_device_set::lock */ > }; Yep, that's essentially what I was suggesting. Thanks, Alex