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[2603:6000:ca08:f320:6401:a7ff:fe72:256d]) by smtp.gmail.com with ESMTPSA id a6sm6121262ili.21.2021.05.19.07.01.53 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 19 May 2021 07:01:54 -0700 (PDT) Mime-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=UTF-8 Cc: "Bjorn Helgaas" , , , , "Alex Williamson" Subject: Re: [PATCH] pci: add NVMe FLR quirk to the SM951 SSD From: "Robert Straw" To: "Christoph Hellwig" Date: Wed, 19 May 2021 07:54:19 -0500 Message-Id: In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-pci@vger.kernel.org On Wed May 19, 2021 at 3:44 AM CDT, Christoph Hellwig wrote: > On Sat, May 15, 2021 at 12:20:05PM -0500, Robert Straw wrote: > While it doesn't matter here, NVMe 1.1 is very much out of data, being > a more than 8 year old specification. The current version is 1.4b, > with NVMe 2.0 about to be released. I can't comment on 2.0, but yes 1.4b has the same aside regarding undefined behavior on the SHST field (on p. 50). The only reason I was looking at 1.1a is because it's specifically listed on the datasheet for the SM951. (The device under test.) > No, we don't. This is a bug particular to a specific implementation. > In fact the whole existing NVMe shutdown before reset quirk is rather > broken and dangerous, as it concurrently accesses the NVMe registers > with the actual driver, which could be trivially triggered through the > sysfs reset attribute. I'm not exactly clear in what way the nvme driver would be racing against= =20 vfio-pci here. (a) vfio-pci is the driver bound in this scenario, and (b) the vfio-pci driver triggers this quirk by issuing an FLR, which is done=20 with the device locked. (e.g: vfio_pci.c:499.) In my testing *without this patch* vfio-pci is still bound to the device=20 for at least 60s after guest shutdown, at which point the FLR times out. After this FLR the device is useless w/o a full reboot of the host.=20 Rebinding it to *either* another guest w/ vfio-pci, or the Linux nvme=20 driver doesn't matter: as the device can no longer be reconfigured. As I understand it: vfio-pci should not blindly issue an FLR to an NVMe cla= ss=20 device w/o obeying the protocol. The protocol seems clear that after a=20 shutdown CC->EN must transition from 1 to 0. (I would argue the guest OS=20 leaving the device in this state is the actual violation of the spec. As=20 I'm unable to change that behavior: having vfio-pci clean up the mess w/=20 this quirk seems to be an adequate workaround.) I am currently testing a version of this patch that only disables the controller if the device has been previously shutdown. I am trying to gauge whether this would be preferable to just blanket-disabling these=20 bugged devices before relinquishing control of them back to the host. > I'd much rather quirk these broken Samsung drivers to not allow > assigning them to VFIO. I'd much rather keep using my storage devices. I will leave the=20 quirk limited to these known-bugged devices.