From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([209.51.188.92]:45615) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1h0oOj-0000VK-8S for qemu-devel@nongnu.org; Mon, 04 Mar 2019 09:16:54 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1h0oOi-0006Bd-5u for qemu-devel@nongnu.org; Mon, 04 Mar 2019 09:16:53 -0500 Date: Mon, 4 Mar 2019 15:16:41 +0100 From: Igor Mammedov Message-ID: <20190304151641.3deefc3b@redhat.com> In-Reply-To: <20190304123908.GK4239@redhat.com> References: <1551454936-205218-1-git-send-email-imammedo@redhat.com> <1551454936-205218-2-git-send-email-imammedo@redhat.com> <20190301154947.GJ21251@redhat.com> <20190301183328.20b63e23@redhat.com> <20190301174806.GN21251@redhat.com> <87va0zcdse.fsf@dusky.pond.sub.org> <20190304132507.39273826@redhat.com> <20190304123908.GK4239@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable Subject: Re: [Qemu-devel] [libvirt] [PATCH 1/2] numa: deprecate 'mem' parameter of '-numa node' option List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: "Daniel P. =?UTF-8?B?QmVycmFuZ8Op?=" Cc: Markus Armbruster , peter.maydell@linaro.org, ehabkost@redhat.com, libvir-list@redhat.com, qemu-devel@nongnu.org, "Dr. David Alan Gilbert" , qemu-arm@nongnu.org, qemu-ppc@nongnu.org, pbonzini@redhat.com, david@gibson.dropbear.id.au, mprivozn@redhat.com On Mon, 4 Mar 2019 12:39:08 +0000 Daniel P. Berrang=C3=A9 wrote: > On Mon, Mar 04, 2019 at 01:25:07PM +0100, Igor Mammedov wrote: > > On Mon, 04 Mar 2019 08:13:53 +0100 > > Markus Armbruster wrote: > > =20 > > > Daniel P. Berrang=C3=A9 writes: > > > =20 > > > > On Fri, Mar 01, 2019 at 06:33:28PM +0100, Igor Mammedov wrote: =20 > > > >> On Fri, 1 Mar 2019 15:49:47 +0000 > > > >> Daniel P. Berrang=C3=A9 wrote: > > > >> =20 > > > >> > On Fri, Mar 01, 2019 at 04:42:15PM +0100, Igor Mammedov wrote: = =20 > > > >> > > The parameter allows to configure fake NUMA topology where gue= st > > > >> > > VM simulates NUMA topology but not actually getting a performa= nce > > > >> > > benefits from it. The same or better results could be achieved > > > >> > > using 'memdev' parameter. In light of that any VM that uses NU= MA > > > >> > > to get its benefits should use 'memdev' and to allow transition > > > >> > > initial RAM to device based model, deprecate 'mem' parameter as > > > >> > > its ad-hoc partitioning of initial RAM MemoryRegion can't be > > > >> > > translated to memdev based backend transparently to users and = in > > > >> > > compatible manner (migration wise). > > > >> > >=20 > > > >> > > That will also allow to clean up a bit our numa code, leaving = only > > > >> > > 'memdev' impl. in place and several boards that use node_mem > > > >> > > to generate FDT/ACPI description from it. =20 > > > >> >=20 > > > >> > Can you confirm that the 'mem' and 'memdev' parameters to -numa > > > >> > are 100% live migration compatible in both directions ? Libvirt > > > >> > would need this to be the case in order to use the 'memdev' synt= ax > > > >> > instead. =20 > > > >> Unfortunately they are not migration compatible in any direction, > > > >> if it where possible to translate them to each other I'd alias 'me= m' > > > >> to 'memdev' without deprecation. The former sends over only one > > > >> MemoryRegion to target, while the later sends over several (one per > > > >> memdev). =20 > > > > > > > > If we can't migration from one to the other, then we can not deprec= ate > > > > the existing 'mem' syntax. Even if libvirt were to provide a config > > > > option to let apps opt-in to the new syntax, we need to be able to > > > > support live migration of existing running VMs indefinitely. Effect= ively > > > > this means we need the to keep 'mem' support forever, or at least s= uch > > > > a long time that it effectively means forever. > > > > > > > > So I think this patch has to be dropped & replaced with one that > > > > simply documents that memdev syntax is preferred. =20 > > >=20 > > > We have this habit of postulating absolutes like "can not deprecate" > > > instead of engaging with the tradeoffs. We need to kick it. > > >=20 > > > So let's have an actual look at the tradeoffs. > > >=20 > > > We don't actually "support live migration of existing running VMs > > > indefinitely". > > >=20 > > > We support live migration to any newer version of QEMU that still > > > supports the machine type. > > >=20 > > > We support live migration to any older version of QEMU that already > > > supports the machine type and all the devices the machine uses. > > >=20 > > > Aside: "support" is really an honest best effort here. If you rely on > > > it, use a downstream that puts in the (substantial!) QA work real > > > support takes. > > >=20 > > > Feature deprecation is not a contract to drop the feature after two > > > releases, or even five. It's a formal notice that users of the featu= re > > > should transition to its replacement in an orderly manner. > > >=20 > > > If I understand Igor correctly, all users should transition away from > > > outdated NUMA configurations at least for new VMs in an orderly manne= r. =20 > > Yes, we can postpone removing options until there are machines type > > versions that were capable to use it (unfortunate but probably=20 > > unavoidable unless there is a migration trick to make transition > > transparent) but that should not stop us from disabling broken > > options on new machine types at least. > >=20 > > This series can serve as formal notice with follow up disabling of > > deprecated options for new machine types. (As Thomas noted, just warnin= gs > > do not work and users continue to use broken features regardless whether > > they are don't know about issues or aware of it [*]) > >=20 > > Hence suggested deprecation approach and enforced rejection of legacy > > numa options for new machine types in 2 releases so users would stop > > using them eventually. =20 >=20 > When we deprecate something, we need to have a way for apps to use the > new alternative approach *at the same time*. So even if we only want to > deprecate for new machine types, we still have to first solve the problem > of how mgmt apps will introspect QEMU to learn which machine types expect > the new options. I'm not aware any mechanism to introspect machine type options (existing or something being developed). Are/were there any ideas about it that were discussed in the past? Aside from developing a new mechanism what are alternative approaches? I mean when we delete deprecated CLI option, how it's solved on libvirt side currently? For example I don't see anything introspection related when we have been removing deprecated options recently. More exact question specific to this series usecase, how libvirt decides when to use -numa node,memdev or not currently? >=20 > Regards, > Daniel