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Tsirkin" Subject: Re: [PATCH] pci: Refuse to hotplug PCI Devices when the Guest OS is not ready Message-ID: <20201023192755.1845b060@redhat.com> In-Reply-To: <20201023115029-mutt-send-email-mst@kernel.org> References: <20201022080354-mutt-send-email-mst@kernel.org> <20201022235632.7f69ddc9@yekko.fritz.box> <20201022100028-mutt-send-email-mst@kernel.org> <20201022102857-mutt-send-email-mst@kernel.org> <20201022110016-mutt-send-email-mst@kernel.org> <20201023144901.5bd908a1@yekko.fritz.box> <20201023115029-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=imammedo@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable Received-SPF: pass client-ip=216.205.24.124; envelope-from=imammedo@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/10/23 01:44:00 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Libvirt Mailing List , David Gibson , Julia Suvorova , qemu devel list Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Fri, 23 Oct 2020 11:54:40 -0400 "Michael S. Tsirkin" wrote: > On Fri, Oct 23, 2020 at 09:47:14AM +0300, Marcel Apfelbaum wrote: > > Hi David, > >=20 > > On Fri, Oct 23, 2020 at 6:49 AM David Gibson wrote= : > >=20 > > On Thu, 22 Oct 2020 11:01:04 -0400 > > "Michael S. Tsirkin" wrote: > > =20 > > > On Thu, Oct 22, 2020 at 05:50:51PM +0300, Marcel Apfelbaum wrote: > > >=C3=83=E2=80=9A=C3=82=C2=A0 [...]=C3=83=E2=80=9A=C3=82=C2=A0 > > > > > > Right. After detecting just failing unconditionally it a bit too > > > simplistic IMHO. =20 > >=20 > > There's also another factor here, which I thought I'd mentioned > > already, but looks like I didn't: I think we're still missing some > > details in what's going on. > >=20 > > The premise for this patch is that plugging while the indicator is = in > > transition state is allowed to fail in any way on the guest side.= =C3=83=E2=80=9A=C3=82=C2=A0 I > > don't think that's a reasonable interpretation, because it's unwork= able > > for physical hotplug.=C3=83=E2=80=9A=C3=82=C2=A0 If the indicator s= tarts blinking while you're in > > the middle of shoving a card in, you'd be in trouble. > >=20 > > So, what I'm assuming here is that while "don't plug while blinking= " is > > the instruction for the operator to obey as best they can, on the g= uest > > side the rule has to be "start blinking, wait a while and by the ti= me > > you leave blinking state again, you can be confident any plugs or > > unplugs have completed".=C3=83=E2=80=9A=C3=82=C2=A0 Obviously still= racy in the strict computer > > science sense, but about the best you can do with slow humans in th= e > > mix. > >=20 > > So, qemu should of course endeavour to follow that rule as though i= t > > was a human operator on a physical machine and not plug when the > > indicator is blinking.=C3=83=E2=80=9A=C3=82=C2=A0 *But* the qemu pl= ug will in practice be fast > > enough that if we're hitting real problems here, it suggests the gu= est > > is still doing something wrong. > >=20 > >=20 > > I personally think there is a little bit of over-engineering=C3=83=E2= =80=9A=C3=82=C2=A0here. > > Let's start with the spec: > >=20 > > =C3=83=E2=80=9A=C3=82=C2=A0 =C3=83=E2=80=9A=C3=82=C2=A0 Power Indicator= Blinking > > =C3=83=E2=80=9A=C3=82=C2=A0 =C3=83=E2=80=9A=C3=82=C2=A0 A blinking Powe= r Indicator indicates that the slot is powering up or > > powering down and that > > =C3=83=E2=80=9A=C3=82=C2=A0 =C3=83=E2=80=9A=C3=82=C2=A0 insertion or re= moval of the adapter is not permitted. > >=20 > > What exactly is an interpretation here? > > As you stated, the races are theoretical, the whole point of the indica= tor > > is to let the operator know he can't plug the device just yet. > >=20 > > I understand it would be more user friendly if the QEMU would wait inte= rnally > > for the > > blinking to end, but the whole point of the indicator is to let the ope= rator=C3=83=E2=80=9A=C3=82=C2=A0 > > (human or machine) > > know they can't plug the device at a specific time. > > Should QEMU take the responsibility=C3=83=E2=80=9A=C3=82=C2=A0of the op= erator? Is it even correct? > >=20 > > Even if we would want such a feature, how is it related to this patch? > > The patch simply refuses to start a hotplug operation when it knows it = will not > > succeed.=C3=83=E2=80=9A=C3=82=C2=A0 > > =C3=83=E2=80=9A=C3=82=C2=A0 > > Another way that would make sense to me would be=C3=83=E2=80=9A=C3=82= =C2=A0 is a new QEMU interface other > > than > > "add_device", let's say "adding_device_allowed", that would return true= if the > > hotplug is allowed > > at this point of time. (I am aware of the theoretical races)=C3=83=E2= =80=9A=C3=82=C2=A0 =20 >=20 > Rather than adding_device_allowed, something like "query slot" > might be helpful for debugging. That would help user figure out > e.g. why isn't device visible without any races. Would be new command useful tough? What we end up is broken guest (if I read commit message right) and a user who has no idea if=20 device_add was successful or not. So what user should do in this case - wait till it explodes? - can user remove it or it would be stuck there forever? - poll slot before hotplug, manually? (if this is the case then failing device_add cleanly doesn't sound bad, it looks similar to another error we have "/* Check if hot-plug is disabled= on the slot */" in pcie_cap_slot_pre_plug_cb) CCing libvirt, as it concerns not only QEMU. >=20 > > The above will at least mimic the mechanics of the pyhs=C3=83=E2=80=9A= =C3=82=C2=A0world.=C3=83=E2=80=9A=C3=82=C2=A0 The operator > > looks at the indicator, > > the management software checks if adding the device is allowed. > > Since it is a corner case I would prefer the device_add to fail rather = than > > introducing a new interface, > > but that's just me. > >=20 > > Thanks, > > Marcel > > =20 >=20 > I think we want QEMU management interface to be reasonably > abstract and agnostic if possible. Pushing knowledge of hardware > detail to management will just lead to pain IMHO. > We supported device_add which practically never fails for years, For CPUs and RAM, device_add can fail so maybe management is also prepared to handle errors on PCI hotplug path. > at this point it's easier to keep supporting it than > change all users ... >=20 >=20 > >=20 > > -- > > David Gibson > > Principal Software Engineer, Virtualization, Red Hat > > =20 >=20 >=20