Messages in this thread | | | From | Suzuki K Poulose <> | Subject | Re: [PATCH v5 18/18] kvm: arm64: Allow tuning the physical address size for VM | Date | Tue, 18 Sep 2018 16:16:27 +0100 |
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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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