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    SubjectRe: [PATCH v5 18/18] kvm: arm64: Allow tuning the physical address size for VM
    Date
    Hi Peter,

    On 18/09/2018 02:55, Peter Maydell wrote:
    > On 17 September 2018 at 11:41, Suzuki K Poulose <suzuki.poulose@arm.com> wrote:
    >> --- a/Documentation/virtual/kvm/api.txt
    >> +++ b/Documentation/virtual/kvm/api.txt
    >> @@ -122,6 +122,14 @@ the default trap & emulate implementation (which changes the virtual
    >> memory layout to fit in user mode), check KVM_CAP_MIPS_VZ and use the
    >> flag KVM_VM_MIPS_VZ.
    >>
    >> +To configure the physical address space size for a VM (IPA size) on arm64,
    >> +check KVM_CAP_ARM_VM_PHYS_SHIFT (which returns the maximum limit for the
    >> +IPA shift) and use KVM_VM_TYPE_ARM_PHYS_SHIFT(PHYS_SHIFT). Bits[7-0] of the
    >> +machine type has been reserved for specifying the PHYS_SHIFT.
    >> +The supported range is [32...IPA_LIMIT], where IPA_LIMIT could be
    >> +identified by checking KVM_CAP_ARM_VM_PHYS_SHIFT. For backward compatibility
    >> +a value of 0 selects 40bits.
    >> +
    >
    > Given this as the API documentation, I don't think I could figure out
    > what I as a userspace user of it need to do without looking at the
    > kernel code. Could I ask you to expand it so that it is a bit less
    > terse and a bit more detailed? (For instance, what is a PHYS_SHIFT
    > and why do I have to specify it rather than just telling the kernel
    > I want a 48 bit guest address space?)

    Thanks for the feedback. I acknowledge that the documentation is not
    quite clear for a userspace user. How about:

    "To configure the physical address space size for a VM (IPA size) on arm64,
    check KVM_CAP_ARM_VM_IPA_SIZE and use KVM_VM_TYPE_ARM_IPA_SIZE(IPA_Bits)
    as the argument to KVM_CREATE_VM, where IPA_Bits is the maximum width of
    any physical address used by the VM. The IPA_Bits is encoded in Bits[7-0]
    of the machine type, and must be one of { 0, 32, ... , Host_IPA_Limit },
    where :
    1) IPA_Bits = 0 implies 40bits IPA (for backward compatibility)
    2) Host_IPA_Limit is the maximum limit for IPA_Bits on the host, which is
    dependent on the CPU capability and the host kernel configuration.
    This can be detected by checking the extension KVM_CAP_ARM_VM_IPA_SIZE
    "

    note: I have renamed the KVM_CAP_ARM_VM_PHYS_SHIFT => KVM_CAP_ARM_VM_IPA_SIZE
    above and can update the code, if the latter is better.

    Suzuki

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