From: "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com> To: Dan Williams <dan.j.williams@intel.com> Cc: linuxppc-dev <linuxppc-dev@lists.ozlabs.org>, Michael Ellerman <mpe@ellerman.id.au>, linux-nvdimm <linux-nvdimm@lists.01.org>, alistair@popple.id.au Subject: Re: [PATCH v2 3/5] libnvdimm/nvdimm/flush: Allow architecture to override the flush barrier Date: Tue, 19 May 2020 19:22:39 +0530 [thread overview] Message-ID: <87d070f2vs.fsf@linux.ibm.com> (raw) In-Reply-To: <CAPcyv4g+oE305Q5bYWkNBKFifB9c0TZo6+hqFQnqiFqU5QFrhQ@mail.gmail.com> Dan Williams <dan.j.williams@intel.com> writes: > On Mon, May 18, 2020 at 10:30 PM Aneesh Kumar K.V > <aneesh.kumar@linux.ibm.com> wrote: ... >> Applications using new instructions will behave as expected when running >> on P8 and P9. Only future hardware will differentiate between 'dcbf' and >> 'dcbfps' > > Right, this is the problem. Applications using new instructions behave > as expected, the kernel has been shipping of_pmem and papr_scm for > several cycles now, you're saying that the DAX applications written > against those platforms are going to be broken on P8 and P9? The expecation is that both kernel and userspace would get upgraded to use the new instruction before actual persistent memory devices are made available. > >> > I'm thinking the kernel >> > should go as far as to disable DAX operation by default on new >> > hardware until userspace asserts that it is prepared to switch to the >> > new implementation. Is there any other way to ensure the forward >> > compatibility of deployed ppc64 DAX applications? >> >> AFAIU there is no released persistent memory hardware on ppc64 platform >> and we need to make sure before applications get enabled to use these >> persistent memory devices, they should switch to use the new >> instruction? > > Right, I want the kernel to offer some level of safety here because > everything you are describing sounds like a flag day conversion. Am I > misreading? Is there some other gate that prevents existing users of > of_pmem and papr_scm from having their expectations violated when > running on P8 / P9 hardware? Maybe there's tighter ecosystem control > that I'm just not familiar with, I'm only going off the fact that the > kernel has shipped a non-zero number of NVDIMM drivers that build with > ARCH=ppc64 for several cycles. If we are looking at adding changes to kernel that will prevent a kernel from running on newer hardware in a specific case, we could as well take the changes to get the kernel use the newer instructions right? But I agree with your concern that if we have older kernel/applications that continue to use `dcbf` on future hardware we will end up having issues w.r.t powerfail consistency. The plan is what you outlined above as tighter ecosystem control. Considering we don't have a pmem device generally available, we get both kernel and userspace upgraded to use these new instructions before such a device is made available. -aneesh _______________________________________________ Linux-nvdimm mailing list -- linux-nvdimm@lists.01.org To unsubscribe send an email to linux-nvdimm-leave@lists.01.org
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From: "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com> To: Dan Williams <dan.j.williams@intel.com> Cc: alistair@popple.id.au, linuxppc-dev <linuxppc-dev@lists.ozlabs.org>, linux-nvdimm <linux-nvdimm@lists.01.org> Subject: Re: [PATCH v2 3/5] libnvdimm/nvdimm/flush: Allow architecture to override the flush barrier Date: Tue, 19 May 2020 19:22:39 +0530 [thread overview] Message-ID: <87d070f2vs.fsf@linux.ibm.com> (raw) In-Reply-To: <CAPcyv4g+oE305Q5bYWkNBKFifB9c0TZo6+hqFQnqiFqU5QFrhQ@mail.gmail.com> Dan Williams <dan.j.williams@intel.com> writes: > On Mon, May 18, 2020 at 10:30 PM Aneesh Kumar K.V > <aneesh.kumar@linux.ibm.com> wrote: ... >> Applications using new instructions will behave as expected when running >> on P8 and P9. Only future hardware will differentiate between 'dcbf' and >> 'dcbfps' > > Right, this is the problem. Applications using new instructions behave > as expected, the kernel has been shipping of_pmem and papr_scm for > several cycles now, you're saying that the DAX applications written > against those platforms are going to be broken on P8 and P9? The expecation is that both kernel and userspace would get upgraded to use the new instruction before actual persistent memory devices are made available. > >> > I'm thinking the kernel >> > should go as far as to disable DAX operation by default on new >> > hardware until userspace asserts that it is prepared to switch to the >> > new implementation. Is there any other way to ensure the forward >> > compatibility of deployed ppc64 DAX applications? >> >> AFAIU there is no released persistent memory hardware on ppc64 platform >> and we need to make sure before applications get enabled to use these >> persistent memory devices, they should switch to use the new >> instruction? > > Right, I want the kernel to offer some level of safety here because > everything you are describing sounds like a flag day conversion. Am I > misreading? Is there some other gate that prevents existing users of > of_pmem and papr_scm from having their expectations violated when > running on P8 / P9 hardware? Maybe there's tighter ecosystem control > that I'm just not familiar with, I'm only going off the fact that the > kernel has shipped a non-zero number of NVDIMM drivers that build with > ARCH=ppc64 for several cycles. If we are looking at adding changes to kernel that will prevent a kernel from running on newer hardware in a specific case, we could as well take the changes to get the kernel use the newer instructions right? But I agree with your concern that if we have older kernel/applications that continue to use `dcbf` on future hardware we will end up having issues w.r.t powerfail consistency. The plan is what you outlined above as tighter ecosystem control. Considering we don't have a pmem device generally available, we get both kernel and userspace upgraded to use these new instructions before such a device is made available. -aneesh
next prev parent reply other threads:[~2020-05-19 13:52 UTC|newest] Thread overview: 43+ messages / expand[flat|nested] mbox.gz Atom feed top 2020-05-13 3:47 [PATCH v2 1/5] powerpc/pmem: Add new instructions for persistent storage and sync Aneesh Kumar K.V 2020-05-13 3:47 ` Aneesh Kumar K.V 2020-05-13 3:47 ` [PATCH v2 2/5] powerpc/pmem: Add flush routines using new pmem store and sync instruction Aneesh Kumar K.V 2020-05-13 3:47 ` Aneesh Kumar K.V 2020-05-13 3:47 ` [PATCH v2 3/5] libnvdimm/nvdimm/flush: Allow architecture to override the flush barrier Aneesh Kumar K.V 2020-05-13 3:47 ` Aneesh Kumar K.V 2020-05-13 16:14 ` Dan Williams 2020-05-13 16:14 ` Dan Williams 2020-05-19 5:30 ` Aneesh Kumar K.V 2020-05-19 5:30 ` Aneesh Kumar K.V 2020-05-19 7:09 ` Dan Williams 2020-05-19 7:09 ` Dan Williams 2020-05-19 13:52 ` Aneesh Kumar K.V [this message] 2020-05-19 13:52 ` Aneesh Kumar K.V 2020-05-19 18:59 ` Dan Williams 2020-05-19 18:59 ` Dan Williams 2020-05-20 18:43 ` Aneesh Kumar K.V 2020-05-20 18:43 ` Aneesh Kumar K.V 2020-05-21 14:38 ` Jeff Moyer 2020-05-21 14:38 ` Jeff Moyer 2020-05-21 17:02 ` Aneesh Kumar K.V 2020-05-21 17:02 ` Aneesh Kumar K.V 2020-05-21 18:25 ` Dan Williams 2020-05-21 18:25 ` Dan Williams 2020-05-21 18:52 ` Mikulas Patocka 2020-05-21 18:52 ` Mikulas Patocka 2020-05-22 9:31 ` Michal Suchánek 2020-05-22 9:31 ` Michal Suchánek 2020-05-22 10:08 ` Aneesh Kumar K.V 2020-05-22 10:08 ` Aneesh Kumar K.V 2020-05-22 13:01 ` Mikulas Patocka 2020-05-22 13:01 ` Mikulas Patocka 2020-06-26 10:20 ` Michal Suchánek 2020-06-26 10:20 ` Michal Suchánek 2020-05-21 18:34 ` Dan Williams 2020-05-21 18:34 ` Dan Williams 2020-05-13 3:47 ` [PATCH v2 4/5] powerpc/pmem/of_pmem: Update of_pmem to use the new barrier instruction Aneesh Kumar K.V 2020-05-13 3:47 ` Aneesh Kumar K.V 2020-05-13 6:44 ` kbuild test robot 2020-05-13 6:44 ` kbuild test robot 2020-05-13 6:44 ` kbuild test robot 2020-05-13 3:47 ` [PATCH v2 5/5] powerpc/pmem: Avoid the barrier in flush routines Aneesh Kumar K.V 2020-05-13 3:47 ` Aneesh Kumar K.V
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