From: Steven Price <email@example.com> To: Andrew Jones <firstname.lastname@example.org> Cc: "Mark Rutland" <email@example.com>, firstname.lastname@example.org, "Radim Krčmář" <email@example.com>, "Marc Zyngier" <firstname.lastname@example.org>, "Suzuki K Pouloze" <email@example.com>, firstname.lastname@example.org, "Russell King" <email@example.com>, firstname.lastname@example.org, "James Morse" <email@example.com>, "Julien Thierry" <firstname.lastname@example.org>, "Catalin Marinas" <email@example.com>, "Paolo Bonzini" <firstname.lastname@example.org>, "Will Deacon" <email@example.com>, firstname.lastname@example.org, email@example.com Subject: Re: [PATCH v4 01/10] KVM: arm64: Document PV-time interface Date: Wed, 4 Sep 2019 16:07:39 +0100 [thread overview] Message-ID: <firstname.lastname@example.org> (raw) In-Reply-To: <email@example.com> On 04/09/2019 15:22, Andrew Jones wrote: > On Wed, Sep 04, 2019 at 02:55:15PM +0100, Steven Price wrote: >> On 02/09/2019 13:52, Andrew Jones wrote: >>> On Fri, Aug 30, 2019 at 04:25:08PM +0100, Steven Price wrote: >>>> On 30/08/2019 15:47, Andrew Jones wrote: >>>>> On Fri, Aug 30, 2019 at 09:42:46AM +0100, Steven Price wrote: >> [...] >>>>>> + Return value: (int32) : NOT_SUPPORTED (-1) or SUCCESS (0) if the relevant >>>>>> + PV-time feature is supported by the hypervisor. >>>>>> + >>>>>> +PV_TIME_ST >>>>>> + Function ID: (uint32) : 0xC5000022 >>>>>> + Return value: (int64) : IPA of the stolen time data structure for this >>>>>> + VCPU. On failure: >>>>>> + NOT_SUPPORTED (-1) >>>>>> + >>>>>> +The IPA returned by PV_TIME_ST should be mapped by the guest as normal memory >>>>>> +with inner and outer write back caching attributes, in the inner shareable >>>>>> +domain. A total of 16 bytes from the IPA returned are guaranteed to be >>>>>> +meaningfully filled by the hypervisor (see structure below). >>>>>> + >>>>>> +PV_TIME_ST returns the structure for the calling VCPU. >>>>>> + >>>>>> +Stolen Time >>>>>> +----------- >>>>>> + >>>>>> +The structure pointed to by the PV_TIME_ST hypercall is as follows: >>>>>> + >>>>>> + Field | Byte Length | Byte Offset | Description >>>>>> + ----------- | ----------- | ----------- | -------------------------- >>>>>> + Revision | 4 | 0 | Must be 0 for version 0.1 >>>>>> + Attributes | 4 | 4 | Must be 0 >>>>> >>>>> The above fields don't appear to be exposed to userspace in anyway. How >>>>> will we handle migration from one KVM with one version of the structure >>>>> to another? >>>> >>>> Interesting question. User space does have access to them now it is >>>> providing the memory, but it's not exactly an easy method. In particular >>>> user space has no (simple) way of probing the kernel's supported version. >>>> >>>> I guess one solution would be to add an extra attribute on the VCPU >>>> which would provide the revision information. The current kernel would >>>> then reject any revision other than 0, but this could then be extended >>>> to support other revision numbers in the future. >>>> >>>> Although there's some logic in saying we could add the extra attribute >>>> when(/if) there is a new version. Future kernels would then be expected >>>> to use the current version unless user space explicitly set the new >>>> attribute. >>>> >>>> Do you feel this is something that needs to be addressed now, or can it >>>> be deferred until another version is proposed? >>> >>> Assuming we'll want userspace to have the option of choosing version=0, >>> and that we're fine with version=0 being the implicit choice, when nothing >>> is selected, then I guess it can be left as is for now. If, OTOH, we just >>> want migration to fail when attempting to migrate to another host with >>> an incompatible stolen-time structure (i.e. version=0 is not selectable >>> on hosts that implement later versions), then we should expose the version >>> in some way now. Perhaps a VCPU's "PV config" should be described in a >>> set of pseudo registers? >> >> I wouldn't have thought making migration fail if/when the host upgrades >> to a new version would be particularly helpful - we'd want to provide >> backwards compatibility. In particular for the suspend/resume case (I >> want to be able to save my VM to disk, upgrade the host kernel and then >> resume the VM). >> >> The only potential issue I see is the implicit "version=0 if not >> specified". That seems solvable by rejecting setting the stolen time >> base address if no version has been specified and the host kernel >> doesn't support version=0. > > I think that's the same failure I was trying avoid by failing the > migration instead. Maybe it's equivalent to fail at this vcpu-ioctl > time though? Yes this is effectively the same failure. But since we require the vcpu-ioctl to enable stolen time this gives an appropriate place to fail. Indeed this is the failure if migrating from a host with these patches to one running an existing kernel with no stolen time support. Steve
next prev parent reply other threads:[~2019-09-04 15:07 UTC|newest] Thread overview: 31+ messages / expand[flat|nested] mbox.gz Atom feed top 2019-08-30 8:42 [PATCH v4 00/10] arm64: Stolen time support Steven Price 2019-08-30 8:42 ` [PATCH v4 01/10] KVM: arm64: Document PV-time interface Steven Price 2019-08-30 14:47 ` Andrew Jones 2019-08-30 15:25 ` Steven Price 2019-09-02 12:52 ` Andrew Jones 2019-09-04 13:55 ` Steven Price 2019-09-04 14:22 ` Andrew Jones 2019-09-04 15:07 ` Steven Price [this message] 2019-08-30 8:42 ` [PATCH v4 02/10] KVM: arm/arm64: Factor out hypercall handling from PSCI code Steven Price 2019-09-02 7:06 ` kbuild test robot 2019-09-04 15:05 ` Steven Price 2019-09-02 13:07 ` kbuild test robot 2019-08-30 8:42 ` [PATCH v4 03/10] KVM: arm64: Implement PV_FEATURES call Steven Price 2019-08-30 8:42 ` [PATCH v4 04/10] KVM: Implement kvm_put_guest() Steven Price 2019-08-30 8:42 ` [PATCH v4 05/10] KVM: arm64: Support stolen time reporting via shared structure Steven Price 2019-08-30 9:42 ` Christoffer Dall 2019-08-30 9:52 ` Steven Price 2019-09-03 9:14 ` Zenghui Yu 2019-09-04 15:53 ` Steven Price 2019-08-30 8:42 ` [PATCH v4 06/10] KVM: Allow kvm_device_ops to be const Steven Price 2019-08-30 8:42 ` [PATCH v4 07/10] KVM: arm64: Provide VCPU attributes for stolen time Steven Price 2019-08-30 10:02 ` Marc Zyngier 2019-08-30 15:04 ` Steven Price 2019-08-30 8:42 ` [PATCH v4 08/10] arm/arm64: Provide a wrapper for SMCCC 1.1 calls Steven Price 2019-08-30 8:42 ` [PATCH v4 09/10] arm/arm64: Make use of the SMCCC 1.1 wrapper Steven Price 2019-08-30 8:42 ` [PATCH v4 10/10] arm64: Retrieve stolen time as paravirtualized guest Steven Price 2019-09-03 8:47 ` Andrew Jones 2019-09-04 16:01 ` Steven Price 2019-09-03 8:03 ` [PATCH v4 00/10] arm64: Stolen time support Andrew Jones 2019-09-03 8:49 ` Andrew Jones 2019-09-04 16:02 ` Steven Price
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