Messages in this thread | | | Date | Fri, 13 Oct 2023 10:29:35 +0100 | From | Will Deacon <> | Subject | Re: [PATCH v1 2/2] KVM: arm64: allow the VM to select DEVICE_* and NORMAL_NC for IO memory |
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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
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