• Sam Bobroff's avatar
    KVM: PPC: Book3S HV: Pack VCORE IDs to access full VCPU ID space · 1e175d2e
    Sam Bobroff authored
    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: default avatarSam Bobroff <sam.bobroff@au1.ibm.com>
    Signed-off-by: default avatarPaul Mackerras <paulus@ozlabs.org>
    1e175d2e
book3s_xive.c 50 KB