From: Will Deacon <will@kernel.org> To: Catalin Marinas <catalin.marinas@arm.com> Cc: Lorenzo Pieralisi <lpieralisi@kernel.org>, Jason Gunthorpe <jgg@nvidia.com>, ankita@nvidia.com, maz@kernel.org, oliver.upton@linux.dev, aniketa@nvidia.com, cjia@nvidia.com, kwankhede@nvidia.com, targupta@nvidia.com, vsethi@nvidia.com, acurrid@nvidia.com, apopple@nvidia.com, jhubbard@nvidia.com, danw@nvidia.com, linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, linux-kernel@vger.kernel.org Subject: Re: [PATCH v1 2/2] KVM: arm64: allow the VM to select DEVICE_* and NORMAL_NC for IO memory Date: Fri, 13 Oct 2023 10:29:35 +0100 [thread overview] Message-ID: <20231013092934.GA13524@willie-the-truck> (raw) In-Reply-To: <ZSgsKSCv-zWgtWkm@arm.com> On Thu, Oct 12, 2023 at 06:26:01PM +0100, Catalin Marinas wrote: > On Thu, Oct 12, 2023 at 03:48:08PM +0100, Will Deacon wrote: > > Claiming back the device also seems strange if the guest has been using > > non-cacheable accesses since I think you could get write merging and > > reordering with subsequent device accesses trying to reset the device. > > True. Not sure we have a good story here (maybe reinvent the DWB barrier ;)). We do have a good story for this part: use Device-nGnRE! > > > So, for now I'd only relax this if we know there's RAM(-like) on the > > > other side and won't trigger some potentially uncontainable errors as a > > > result. > > > > I guess my wider point is that I'm not convinced that non-cacheable is > > actually much better and I think we're going way off the deep end looking > > at what particular implementations do and trying to justify to ourselves > > that non-cacheable is safe, even though it's still a normal memory type > > at the end of the day. > > Is this about Device vs NC or Device/NC vs Normal Cacheable? The > justification for the former has been summarised in Lorenzo's write-up. > How the hardware behaves, it depends a lot on the RAS implementation. > The BSA has some statements but not sure it covers everything. I don't know how to be more clear, but I'm asking why Normal-NC is the right memory type to use. Jason's explanation on the other thread about how it's basically the only option with FWB (with some hand-waving about why Normal-cacheable isn't safe) will have to do, but it needs to be in the commit message. The host maps MMIO with Device-nGnRE. Allowing a guest to map it as Normal surely means the host is going to need additional logic to deal with that? We briefly discussed claiming back a device above and I'm not convinced that the code we have for doing that today will work correctly if the guest has issued a bunch of Normal-NC stores prior to the device being unmapped. Could we change these patches so that the memory type of the stage-1 VMA in the VMM is reflected in the stage-2? In other words, continue to use Device mappings at stage-2 for I/O but relax to Normal-NC if that's how the VMM has it mapped? > Things can go wrong but that's not because Device does anything better. > Given the RAS implementation, external aborts caused on Device memory > (e.g. wrong size access) is uncontainable. For Normal NC it can be > contained (I can dig out the reasoning behind this if you want, IIUC > something to do with not being able to cancel an already issued Device > access since such accesses don't allow speculation due to side-effects; > for Normal NC, it's just about the software not getting the data). I really think these details belong in the commit message. > > Obviously, it's up to Marc and Oliver if they want to do this, but I'm > > wary without an official statement from Arm to say that Normal-NC is > > correct. There's mention of such a statement in the cover letter: > > > > > We hope ARM will publish information helping platform designers > > > follow these guidelines. > > > > but imo we shouldn't merge this without either: > > > > (a) _Architectural_ guidance (as opposed to some random whitepaper or > > half-baked certification scheme). > > Well, you know the story, the architects will probably make it a SoC or > integration issue, PCIe etc., not something that can live in the Arm > ARM. The best we could get is more recommendations in the RAS spec > around containment but not for things that might happen outside the CPU, > e.g. PCIe root complex. The Arm ARM _does_ mention PCI config space when talking about early write acknowledgement, so there's some precedence for providing guidance around which memory types to use. > > - or - > > > > (b) A concrete justification based on the current architecture as to > > why Normal-NC is the right thing to do for KVM. > > To put it differently, we don't have any strong arguments why Device is > the right thing to do. We chose Device based on some understanding > software people had about how the hardware behaves, which apparently > wasn't entirely correct (and summarised by Lorenzo). I think we use Device because that's what the host uses in its stage-1 and mismatched aliases are bad. Will
WARNING: multiple messages have this Message-ID (diff)
From: Will Deacon <will@kernel.org> To: Catalin Marinas <catalin.marinas@arm.com> Cc: Lorenzo Pieralisi <lpieralisi@kernel.org>, Jason Gunthorpe <jgg@nvidia.com>, ankita@nvidia.com, maz@kernel.org, oliver.upton@linux.dev, aniketa@nvidia.com, cjia@nvidia.com, kwankhede@nvidia.com, targupta@nvidia.com, vsethi@nvidia.com, acurrid@nvidia.com, apopple@nvidia.com, jhubbard@nvidia.com, danw@nvidia.com, linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, linux-kernel@vger.kernel.org Subject: Re: [PATCH v1 2/2] KVM: arm64: allow the VM to select DEVICE_* and NORMAL_NC for IO memory Date: Fri, 13 Oct 2023 10:29:35 +0100 [thread overview] Message-ID: <20231013092934.GA13524@willie-the-truck> (raw) In-Reply-To: <ZSgsKSCv-zWgtWkm@arm.com> On Thu, Oct 12, 2023 at 06:26:01PM +0100, Catalin Marinas wrote: > On Thu, Oct 12, 2023 at 03:48:08PM +0100, Will Deacon wrote: > > Claiming back the device also seems strange if the guest has been using > > non-cacheable accesses since I think you could get write merging and > > reordering with subsequent device accesses trying to reset the device. > > True. Not sure we have a good story here (maybe reinvent the DWB barrier ;)). We do have a good story for this part: use Device-nGnRE! > > > So, for now I'd only relax this if we know there's RAM(-like) on the > > > other side and won't trigger some potentially uncontainable errors as a > > > result. > > > > I guess my wider point is that I'm not convinced that non-cacheable is > > actually much better and I think we're going way off the deep end looking > > at what particular implementations do and trying to justify to ourselves > > that non-cacheable is safe, even though it's still a normal memory type > > at the end of the day. > > Is this about Device vs NC or Device/NC vs Normal Cacheable? The > justification for the former has been summarised in Lorenzo's write-up. > How the hardware behaves, it depends a lot on the RAS implementation. > The BSA has some statements but not sure it covers everything. I don't know how to be more clear, but I'm asking why Normal-NC is the right memory type to use. Jason's explanation on the other thread about how it's basically the only option with FWB (with some hand-waving about why Normal-cacheable isn't safe) will have to do, but it needs to be in the commit message. The host maps MMIO with Device-nGnRE. Allowing a guest to map it as Normal surely means the host is going to need additional logic to deal with that? We briefly discussed claiming back a device above and I'm not convinced that the code we have for doing that today will work correctly if the guest has issued a bunch of Normal-NC stores prior to the device being unmapped. Could we change these patches so that the memory type of the stage-1 VMA in the VMM is reflected in the stage-2? In other words, continue to use Device mappings at stage-2 for I/O but relax to Normal-NC if that's how the VMM has it mapped? > Things can go wrong but that's not because Device does anything better. > Given the RAS implementation, external aborts caused on Device memory > (e.g. wrong size access) is uncontainable. For Normal NC it can be > contained (I can dig out the reasoning behind this if you want, IIUC > something to do with not being able to cancel an already issued Device > access since such accesses don't allow speculation due to side-effects; > for Normal NC, it's just about the software not getting the data). I really think these details belong in the commit message. > > Obviously, it's up to Marc and Oliver if they want to do this, but I'm > > wary without an official statement from Arm to say that Normal-NC is > > correct. There's mention of such a statement in the cover letter: > > > > > We hope ARM will publish information helping platform designers > > > follow these guidelines. > > > > but imo we shouldn't merge this without either: > > > > (a) _Architectural_ guidance (as opposed to some random whitepaper or > > half-baked certification scheme). > > Well, you know the story, the architects will probably make it a SoC or > integration issue, PCIe etc., not something that can live in the Arm > ARM. The best we could get is more recommendations in the RAS spec > around containment but not for things that might happen outside the CPU, > e.g. PCIe root complex. The Arm ARM _does_ mention PCI config space when talking about early write acknowledgement, so there's some precedence for providing guidance around which memory types to use. > > - or - > > > > (b) A concrete justification based on the current architecture as to > > why Normal-NC is the right thing to do for KVM. > > To put it differently, we don't have any strong arguments why Device is > the right thing to do. We chose Device based on some understanding > software people had about how the hardware behaves, which apparently > wasn't entirely correct (and summarised by Lorenzo). I think we use Device because that's what the host uses in its stage-1 and mismatched aliases are bad. Will _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
next prev parent reply other threads:[~2023-10-13 9:29 UTC|newest] Thread overview: 110+ messages / expand[flat|nested] mbox.gz Atom feed top 2023-09-07 18:14 [PATCH v1 0/2] KVM: arm64: support write combining and cachable IO memory in VMs ankita 2023-09-07 18:14 ` ankita 2023-09-07 18:14 ` [PATCH v1 1/2] KVM: arm64: determine memory type from VMA ankita 2023-09-07 18:14 ` ankita 2023-09-07 19:12 ` Jason Gunthorpe 2023-09-07 19:12 ` Jason Gunthorpe 2023-10-05 16:15 ` Catalin Marinas 2023-10-05 16:15 ` Catalin Marinas 2023-10-05 16:54 ` Jason Gunthorpe 2023-10-05 16:54 ` Jason Gunthorpe 2023-10-10 14:25 ` Catalin Marinas 2023-10-10 14:25 ` Catalin Marinas 2023-10-10 15:05 ` Jason Gunthorpe 2023-10-10 15:05 ` Jason Gunthorpe 2023-10-10 17:19 ` Catalin Marinas 2023-10-10 17:19 ` Catalin Marinas 2023-10-10 18:23 ` Jason Gunthorpe 2023-10-10 18:23 ` Jason Gunthorpe 2023-10-11 17:45 ` Catalin Marinas 2023-10-11 17:45 ` Catalin Marinas 2023-10-11 18:38 ` Jason Gunthorpe 2023-10-11 18:38 ` Jason Gunthorpe 2023-10-12 16:16 ` Catalin Marinas 2023-10-12 16:16 ` Catalin Marinas 2024-03-10 3:49 ` Ankit Agrawal 2024-03-10 3:49 ` Ankit Agrawal 2024-03-19 13:38 ` Jason Gunthorpe 2024-03-19 13:38 ` Jason Gunthorpe 2023-10-23 13:20 ` Shameerali Kolothum Thodi 2023-10-23 13:20 ` Shameerali Kolothum Thodi 2023-09-07 18:14 ` [PATCH v1 2/2] KVM: arm64: allow the VM to select DEVICE_* and NORMAL_NC for IO memory ankita 2023-09-07 18:14 ` ankita 2023-09-08 16:40 ` Catalin Marinas 2023-09-08 16:40 ` Catalin Marinas 2023-09-11 14:57 ` Lorenzo Pieralisi 2023-09-11 14:57 ` Lorenzo Pieralisi 2023-09-11 17:20 ` Jason Gunthorpe 2023-09-11 17:20 ` Jason Gunthorpe 2023-09-13 15:26 ` Lorenzo Pieralisi 2023-09-13 15:26 ` Lorenzo Pieralisi 2023-09-13 18:54 ` Jason Gunthorpe 2023-09-13 18:54 ` Jason Gunthorpe 2023-09-26 8:31 ` Lorenzo Pieralisi 2023-09-26 8:31 ` Lorenzo Pieralisi 2023-09-26 12:25 ` Jason Gunthorpe 2023-09-26 12:25 ` Jason Gunthorpe 2023-09-26 13:52 ` Catalin Marinas 2023-09-26 13:52 ` Catalin Marinas 2023-09-26 16:12 ` Lorenzo Pieralisi 2023-09-26 16:12 ` Lorenzo Pieralisi 2023-10-05 9:56 ` Lorenzo Pieralisi 2023-10-05 9:56 ` Lorenzo Pieralisi 2023-10-05 11:56 ` Jason Gunthorpe 2023-10-05 11:56 ` Jason Gunthorpe 2023-10-05 14:08 ` Lorenzo Pieralisi 2023-10-05 14:08 ` Lorenzo Pieralisi 2023-10-12 12:35 ` Will Deacon 2023-10-12 12:35 ` Will Deacon 2023-10-12 13:20 ` Jason Gunthorpe 2023-10-12 13:20 ` Jason Gunthorpe 2023-10-12 14:29 ` Lorenzo Pieralisi 2023-10-12 14:29 ` Lorenzo Pieralisi 2023-10-12 13:53 ` Catalin Marinas 2023-10-12 13:53 ` Catalin Marinas 2023-10-12 14:48 ` Will Deacon 2023-10-12 14:48 ` Will Deacon 2023-10-12 15:44 ` Jason Gunthorpe 2023-10-12 15:44 ` Jason Gunthorpe 2023-10-12 16:39 ` Will Deacon 2023-10-12 16:39 ` Will Deacon 2023-10-12 18:36 ` Jason Gunthorpe 2023-10-12 18:36 ` Jason Gunthorpe 2023-10-13 9:29 ` Will Deacon 2023-10-13 9:29 ` Will Deacon 2023-10-12 17:26 ` Catalin Marinas 2023-10-12 17:26 ` Catalin Marinas 2023-10-13 9:29 ` Will Deacon [this message] 2023-10-13 9:29 ` Will Deacon 2023-10-13 13:08 ` Catalin Marinas 2023-10-13 13:08 ` Catalin Marinas 2023-10-13 13:45 ` Jason Gunthorpe 2023-10-13 13:45 ` Jason Gunthorpe 2023-10-19 11:07 ` Catalin Marinas 2023-10-19 11:07 ` Catalin Marinas 2023-10-19 11:51 ` Jason Gunthorpe 2023-10-19 11:51 ` Jason Gunthorpe 2023-10-20 11:21 ` Catalin Marinas 2023-10-20 11:21 ` Catalin Marinas 2023-10-20 11:47 ` Jason Gunthorpe 2023-10-20 11:47 ` Jason Gunthorpe 2023-10-20 14:03 ` Lorenzo Pieralisi 2023-10-20 14:03 ` Lorenzo Pieralisi 2023-10-20 14:28 ` Jason Gunthorpe 2023-10-20 14:28 ` Jason Gunthorpe 2023-10-19 13:35 ` Lorenzo Pieralisi 2023-10-19 13:35 ` Lorenzo Pieralisi 2023-10-13 15:28 ` Lorenzo Pieralisi 2023-10-13 15:28 ` Lorenzo Pieralisi 2023-10-19 11:12 ` Catalin Marinas 2023-10-19 11:12 ` Catalin Marinas 2023-11-09 15:34 ` Lorenzo Pieralisi 2023-11-09 15:34 ` Lorenzo Pieralisi 2023-11-10 14:26 ` Jason Gunthorpe 2023-11-10 14:26 ` Jason Gunthorpe 2023-11-13 0:42 ` Lorenzo Pieralisi 2023-11-13 0:42 ` Lorenzo Pieralisi 2023-11-13 17:41 ` Catalin Marinas 2023-11-13 17:41 ` Catalin Marinas 2023-10-12 12:27 ` Will Deacon 2023-10-12 12:27 ` Will Deacon
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