Re: [PATCH v5 18/18] kvm: arm64: Allow tuning the physical address size for VM

From: Suzuki K Poulose
Date: Tue Sep 18 2018 - 11:16:33 EST

Hi Peter,

On 18/09/2018 02:55, Peter Maydell wrote:
On 17 September 2018 at 11:41, Suzuki K Poulose <suzuki.poulose@xxxxxxx> 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

+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,
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

above and can update the code, if the latter is better.