forked from Minki/linux
1e175d2e07
It is not currently possible to create the full number of possible VCPUs (KVM_MAX_VCPUS) on Power9 with KVM-HV when the guest uses fewer threads per core than its core stride (or "VSMT mode"). This is because the VCORE ID and XIVE offsets grow beyond KVM_MAX_VCPUS even though the VCPU ID is less than KVM_MAX_VCPU_ID. To address this, "pack" the VCORE ID and XIVE offsets by using knowledge of the way the VCPU IDs will be used when there are fewer guest threads per core than the core stride. The primary thread of each core will always be used first. Then, if the guest uses more than one thread per core, these secondary threads will sequentially follow the primary in each core. So, the only way an ID above KVM_MAX_VCPUS can be seen, is if the VCPUs are being spaced apart, so at least half of each core is empty, and IDs between KVM_MAX_VCPUS and (KVM_MAX_VCPUS * 2) can be mapped into the second half of each core (4..7, in an 8-thread core). Similarly, if IDs above KVM_MAX_VCPUS * 2 are seen, at least 3/4 of each core is being left empty, and we can map down into the second and third quarters of each core (2, 3 and 5, 6 in an 8-thread core). Lastly, if IDs above KVM_MAX_VCPUS * 4 are seen, only the primary threads are being used and 7/8 of the core is empty, allowing use of the 1, 5, 3 and 7 thread slots. (Strides less than 8 are handled similarly.) This allows the VCORE ID or offset to be calculated quickly from the VCPU ID or XIVE server numbers, without access to the VCPU structure. [paulus@ozlabs.org - tidied up comment a little, changed some WARN_ONCE to pr_devel, wrapped line, fixed id check.] Signed-off-by: Sam Bobroff <sam.bobroff@au1.ibm.com> Signed-off-by: Paul Mackerras <paulus@ozlabs.org> |
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