Messages in this thread | | | Subject | Re: [PATCH v9 21/22] KVM: s390: CPU model support for AP virtualization | From | Pierre Morel <> | Date | Thu, 23 Aug 2018 10:26:05 +0200 |
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On 23/08/2018 09:48, David Hildenbrand wrote: >>>> >>> I really wonder if we should also export the APXA facility. >> >> Given this comment is made within the context of the >> FACILITIES_KVM_CPUMODEL I might point out that APXA is not >> indicated by a facilities bit. It is indicated by a bit in >> the QCI control block returned from the PQAP(QCI) >> instruction to indicate that APXA is installed on all CPUs. >> >>> We can probe and allow that CPU feature. However, we cannot disable it >>> (as of now). >> >> Given this patch series implements passthrough devices, >> the output of the PQAP(QCI) will always be from a real >> device - i.e., there will be no way to disable it. >> > > see below > >>> >>> We have other CPU features where it is the same case (basically all >>> subfunctions). See kvm_s390_get_processor_subfunc(). We probe them and >>> export them, but support to disable them has never been implemented. >>> >>> On a high level, we could then e.g. deny to start a QEMU guest if APXA >>> is available but has been disabled. (until we know that disabling it >>> actually works - if ever). >>> >>> This helps to catch nasty migration bugs (e.g. APXA suddenly >>> disappearing). Although unlikely, definitely possible. >> >> Migration of AP devices is not supported by this patch series, so this >> should >> not be an issue. > > Might not be a problem now, but could be later. As I said in a different > reply, the CPU model in QEMU does not care about KVM. > > I want the QEMU CPU model and the KVM interfaces to be clean and future > proof. That's why my opinion is to handle PQAP(QCI) just like all the > other "feature blocks" we already have. >
Don't you mix with the TAPQ instruction which has a T bit to specify interception. It indeed is not in the subfunction list even it has a T bit to indicate interception.
TAPQ-t is indicated through the APFT facility.
We can use this bit as an indication of the presence of APXA, the documentation mention that both are implemented together.
regards, Pierre
-- Pierre Morel Linux/KVM/QEMU in Böblingen - Germany
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