On 18 September 2018 at 16:16, Suzuki K Poulose [off-list ref] wrote:

Hi Peter,

On 18/09/2018 02:55, Peter Maydell wrote:

quoted

On 17 September 2018 at 11:41, Suzuki K Poulose [off-list ref]
wrote:

quoted

--- 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?

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)?

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.

I think the new name is definitely clearer, thanks.

-- PMM