On 04/02/18 12:37, Christoffer Dall wrote: > On Sat, Feb 03, 2018 at 11:59:32AM +0000, Marc Zyngier wrote: >> On Fri, 2 Feb 2018 21:17:06 +0100 >> Andrew Jones <drjones@xxxxxxxxxx> wrote: >> >>> On Thu, Feb 01, 2018 at 11:46:47AM +0000, Marc Zyngier wrote: >>>> Although we've implemented PSCI 1.0 and 1.1, nothing can select them >>>> Since all the new PSCI versions are backward compatible, we decide to >>>> default to the latest version of the PSCI implementation. This is no >>>> different from doing a firmware upgrade on KVM. >>>> >>>> But in order to give a chance to hypothetical badly implemented guests >>>> that would have a fit by discovering something other than PSCI 0.2, >>>> let's provide a new API that allows userspace to pick one particular >>>> version of the API. >>>> >>>> This is implemented as a new class of "firmware" registers, where >>>> we expose the PSCI version. This allows the PSCI version to be >>>> save/restored as part of a guest migration, and also set to >>>> any supported version if the guest requires it. >>>> >>>> Signed-off-by: Marc Zyngier <marc.zyngier@xxxxxxx> >>>> --- >>>> Documentation/virtual/kvm/api.txt | 3 +- >>>> Documentation/virtual/kvm/arm/psci.txt | 30 +++++++++++++++ >>>> arch/arm/include/asm/kvm_host.h | 3 ++ >>>> arch/arm/include/uapi/asm/kvm.h | 6 +++ >>>> arch/arm/kvm/guest.c | 13 +++++++ >>>> arch/arm64/include/asm/kvm_host.h | 3 ++ >>>> arch/arm64/include/uapi/asm/kvm.h | 6 +++ >>>> arch/arm64/kvm/guest.c | 14 ++++++- >>>> include/kvm/arm_psci.h | 9 +++++ >>>> virt/kvm/arm/psci.c | 68 +++++++++++++++++++++++++++++++++- >>>> 10 files changed, 151 insertions(+), 4 deletions(-) >>>> create mode 100644 Documentation/virtual/kvm/arm/psci.txt >>>> >>>> diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt >>>> index 57d3ee9e4bde..334905202141 100644 >>>> --- a/Documentation/virtual/kvm/api.txt >>>> +++ b/Documentation/virtual/kvm/api.txt >>>> @@ -2493,7 +2493,8 @@ Possible features: >>>> and execute guest code when KVM_RUN is called. >>>> - KVM_ARM_VCPU_EL1_32BIT: Starts the CPU in a 32bit mode. >>>> Depends on KVM_CAP_ARM_EL1_32BIT (arm64 only). >>>> - - KVM_ARM_VCPU_PSCI_0_2: Emulate PSCI v0.2 for the CPU. >>>> + - KVM_ARM_VCPU_PSCI_0_2: Emulate PSCI v0.2 (or a future revision >>>> + backward compatible with v0.2) for the CPU. >>>> Depends on KVM_CAP_ARM_PSCI_0_2. >>>> - KVM_ARM_VCPU_PMU_V3: Emulate PMUv3 for the CPU. >>>> Depends on KVM_CAP_ARM_PMU_V3. >>>> diff --git a/Documentation/virtual/kvm/arm/psci.txt b/Documentation/virtual/kvm/arm/psci.txt >>>> new file mode 100644 >>>> index 000000000000..aafdab887b04 >>>> --- /dev/null >>>> +++ b/Documentation/virtual/kvm/arm/psci.txt >>>> @@ -0,0 +1,30 @@ >>>> +KVM implements the PSCI (Power State Coordination Interface) >>>> +specification in order to provide services such as CPU on/off, reset >>>> +and power-off to the guest. >>>> + >>>> +The PSCI specification is regularly updated to provide new features, >>>> +and KVM implements these updates if they make sense from a virtualization >>>> +point of view. >>>> + >>>> +This means that a guest booted on two different versions of KVM can >>>> +observe two different "firmware" revisions. This could cause issues if >>>> +a given guest is tied to a particular PSCI revision (unlikely), or if >>>> +a migration causes a different PSCI version to be exposed out of the >>>> +blue to an unsuspecting guest. >>>> + >>>> +In order to remedy this situation, KVM exposes a set of "firmware >>>> +pseudo-registers" that can be manipulated using the GET/SET_ONE_REG >>>> +interface. These registers can be saved/restored by userspace, and set >>>> +to a convenient value if required. >>>> + >>>> +The following register is defined: >>>> + >>>> +* KVM_REG_ARM_PSCI_VERSION: >>>> + >>>> + - Only valid if the vcpu has the KVM_ARM_VCPU_PSCI_0_2 feature set >>>> + (and thus has already been initialized) >>>> + - Returns the current PSCI version on GET_ONE_REG (defaulting to the >>>> + highest PSCI version implemented by KVM and compatible with v0.2) >>>> + - Allows any PSCI version implemented by KVM and compatible with >>>> + v0.2 to be set with SET_ONE_REG >>>> + - Affects the whole VM (even if the register view is per-vcpu) >>> >> >> Hi Drew, >> >> Thanks for looking into this, and for the exhaustive data. >> >>> >>> I've put some more thought and experimentation into this. I think we >>> should change to a vcpu feature bit. The feature bit would be used to >>> force compat mode, v0.2, so KVM would still enable the new PSCI >>> version by default. Below are two tables describing why I think we >>> should switch to something other than a new sysreg, and below those >>> tables are notes as to why I think we should use a vcpu feature. The >>> asterisks in the tables point out behaviors that aren't what we want. >>> While both tables have an asterisk, the sysreg approach's issue is >>> bug. The vcpu feature approach's issue is risk incurred from an >>> unsupported migration, albeit one that is hard to detect without a >>> new machine type. >>> >>> +-----------------------------------------------------------------------+ >>> | sysreg approach | >>> +------------------+-----------+-------+--------------------------------+ >>> | migration | userspace | works | notes | >>> | | change | | | >>> +------------------+-----------+-------+--------------------------------+ >>> | new -> new | NO | YES | Expected | >>> +------------------+-----------+-------+--------------------------------+ >>> | old -> new | NO | YES | PSCI 1.0 is backward compatible| >>> +------------------+-----------+-------+--------------------------------+ >>> | new -> old | NO | NO | Migration fails due to the new | >>> | | | | sysreg. Migration shouldn't | >>> | | | | have been attempted, but no | >>> | | | | way to know without a new | >>> | | | | machine type. | >>> +------------------+-----------+-------+--------------------------------+ >>> | compat -> old | YES | NO* | Even when setting PSCI version | >>> | | | | to 0.2, we add a new sysreg, | >>> | | | | so migration will still fail. | >>> +------------------+-----------+-------+--------------------------------+ >>> | old -> compat | YES | YES | It's OK for the destination to | >>> | | | | support more sysregs than the | >>> | | | | source sends. | >>> +------------------+-----------+-------+--------------------------------+ >>> >>> >>> +-----------------------------------------------------------------------+ >>> | vcpu feature approach | >>> +------------------+-----------+-------+--------------------------------+ >>> | migration | userspace | works | notes | >>> | | change | | | >>> +------------------+-----------+-------+--------------------------------+ >>> | new -> new | NO | YES | Expected | >>> +------------------+-----------+-------+--------------------------------+ >>> | old -> new | NO | YES | PSCI 1.0 is backward compatible| >>> +------------------+-----------+-------+--------------------------------+ >>> | new -> old | NO | YES* | Migrates, but it's not safe | >>> | | | | for the guest kernel, and no | >>> | | | | way to know without a new | >>> | | | | machine type. | >>> +------------------+-----------+-------+--------------------------------+ >>> | compat -> old | YES | YES | Expected | >>> +------------------+-----------+-------+--------------------------------+ >>> | old -> compat | YES | YES | Expected | >>> +------------------+-----------+-------+--------------------------------+ >>> >>> >>> Notes as to why the vcpu feature approach was selected: >>> >>> 1) While this is VM state, and thus a VM control would be a more natural >>> fit, a new vcpu feature bit would be much less new code. We also >>> already have a PSCI vcpu feature bit, so a new one will actually fit >>> quite well. >>> >>> 2) No new state needs to be migrated, as we already migrate the feature >>> bitmap. Unlike, KVM, QEMU doesn't track the max number of features, >>> so bumping it one more in KVM doesn't require a QEMU change. >>> >>> >>> If we switch to a vcpu feature bit, then I think this patch can be >>> replaced with something like this >> >> A couple of remarks: >> >> - My worry with this feature bit is that it is a point fix, and it >> doesn't scale. Come PSCI 1.2 and WORKAROUND_2, what do we do? Add >> another feature bit that says "force to 1.0"? I'd really like >> something we can live with in the long run, and "KVM as firmware" >> needs to be able to evolve without requiring a new userspace >> interface each time we rev it. >> >> - The "compat->old" entry in your sysreg table is not quite fair. In >> the feature table, you teach userspace about the new feature bit. You >> could just as well teach userspace about the new sysreg. Yes, things >> may be different in QEMU, but that's not what we're talking about >> here. >> >> - Allowing a guest to migrate in an unsafe way seems worse than failing >> a migration unexpectedly. Or at least not any better. >> >> To be clear: I'm not dismissing the idea at all, but I want to make sure >> we're not cornering ourselves into an uncomfortable place. >> >> Christoffer, Peter, what are your thoughts on this? >> > > Taking a step back, the only reasons why this patch isn't simply > enabling PSCI v1.0 by default (without any selection method) are that we > (1) want to support guests that complain about PSCI_VERSION != 0.2 > (which isn't completely outside the realm of a reasonable implementation > if you read the description of PSCI_VERSION in the 0.2 spec) and (2) to > provide migration support for guests that call > PSCI_1_0_FN_PSCI_FEATURES. > > If we ignore (1) because we don't know of any guests where this is an > issue, then it's all about (2), migration from "new -> old". > > As far as I can tell, the use case we are worried about here is updating > the kernel (and not QEMU) on half of your data center and then trying to > migrate from the upgraded kernel machine to a legacy (and potentially > variant 2 vulnerable) machine. For this specific move from PSCI 0.2 to > 1.0 with the included mitigation, I don't really think this is an > important use case to support. I'm not so sure. Promising mitigation to a guest, and then seeing that mitigation being silently taken away because we've allowed it to migrate seem bad to me. > In terms of the more general approach to "KVM firmware upgrades" and > migration, I think something like the proposed FW register interface > here should work, but I'm concerned about the lack of opportunity from > userspace to predict a migration failure. But I don't understand why Userspace could predict some of the failure cases, if only by checking that all registers can be restored in a new guest. I'm not sure how viable this is in a data centre type of environment. > this requires a new machine type? Why can't we simply provide a KVM > capability that libvirt etc. can query for? > > Also, is it generally true that we can't expose any additional system > registers from KVM without breaking migration and we don't have any > method to deal with that in userspace and upper layers? If that's true, It is my understanding that each time we add a new sysreg to KVM, migration in QEMU breaks in the new->old direction. > that's a bigger problem in general and something we should work on > trying to solve. If it's not true, then there should be some method to > deal with the FW register already (like capabilities). > > Given the urgency of adding mitigation towards variant 2 which is the > driver for this work, I think we should drop the compat functionality in > this series and work this out later on if needed. I think we can just > tweak the previous patch to enable PSCI 1.0 by default and drop this > patch for the current merge window. I'd be fine with that, as long as we have a clear agreement on the impact of such a move. Thanks, M. -- Jazz is not dead. It just smells funny... _______________________________________________ kvmarm mailing list kvmarm@xxxxxxxxxxxxxxxxxxxxx https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
