Messages in this thread | | | Subject | Re: [PATCH v5 18/18] kvm: arm64: Allow tuning the physical address size for VM | From | Suzuki K Poulose <> | Date | Tue, 18 Sep 2018 17:27:22 +0100 |
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Hi Peter,
On 18/09/18 16:36, Peter Maydell wrote: > On 18 September 2018 at 16:16, Suzuki K Poulose <Suzuki.Poulose@arm.com> wrote: >> 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 >> " > > I think this is still somewhat confusing. In particular, you're > describing both the "ask the kernel what it supports" API and > the "tell the kernel what we want" API in a single sentence > ("...check KVM_CAP_ARM_VM_IPA_SIZE and use KVM_VM_TYPE_ARM_IPA_SIZE..."). > There isn't a length limit on documentation, so why not describe > them both clearly in separate sentences?
Sure, will do. I didn't want to hijack the KVM_CREATE_VM ioctl command section to explain a kvm CAP.
> > Also, can I use any IPA value between 32 and Host_IPA_Limit, or > are only certain values in that range supported (if so, which)?
Any value in the range is supported. I should probably add that "The configured IPA size is different from what is observed by guest in ID_AA64MMFR0_EL1[PARange] to avoid the confusion." So here it goes :
---
"On arm64, the physical address size for a VM (IPA Size limit) is limited to 40bits by default. The limit can be configured if the host supports the extension KVM_CAP_ARM_VM_IPA_SIZE. When supported, use KVM_VM_TYPE_ARM_IPA_SIZE(IPA_Bits) to set the size in the machine type identifier, 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 identifier.
e.g, to configure a guest to use 48bit physical address size :
vm_fd = ioctl(dev_fd, KVM_CREATE_VM, KVM_VM_TYPE_ARM_IPA_SIZE(48));
The requested size (IPA_Bits) must be : 0 - Implies default 40bits (for backward compatibility)
or
N - Implies N bits, where N is a positive integer such that 32 <= N <= Host_IPA_Limit
Host_IPA_Limit is the maximum possible value for IPA_Bits on the host and is dependent on the CPU capability and the kernel configuration. The limit can be retrieved using KVM_CAP_ARM_VM_IPA_SIZE of the KVM_CHECK_EXTENSION ioctl() at run-time.
Please note that configuring the IPA size does not affect the capability exposed by the guest CPUs in ID_AA64MMFR0_EL1[PARange]. It only affects the guest to host physical address (stage2) translations setup by the host. "
Suzuki
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