[Top][All Lists]

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [Qemu-devel] [PATCH v3 15/20] kvm: arm/arm64: Allow tuning the physi

From: Suzuki K Poulose
Subject: Re: [Qemu-devel] [PATCH v3 15/20] kvm: arm/arm64: Allow tuning the physical address size for VM
Date: Tue, 10 Jul 2018 17:38:39 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.8.0

On 09/07/18 14:37, Dave Martin wrote:
On Mon, Jul 09, 2018 at 01:29:42PM +0100, Marc Zyngier wrote:
On 09/07/18 12:23, Dave Martin wrote:
On Fri, Jul 06, 2018 at 05:39:00PM +0100, Suzuki K Poulose wrote:
On 07/06/2018 04:09 PM, Marc Zyngier wrote:
On 06/07/18 14:49, Suzuki K Poulose wrote:
On 04/07/18 23:03, Suzuki K Poulose wrote:
On 07/04/2018 04:51 PM, Will Deacon wrote:
Hi Suzuki,

On Fri, Jun 29, 2018 at 12:15:35PM +0100, Suzuki K Poulose wrote:
Allow specifying the physical address size for a new VM via
the kvm_type argument for KVM_CREATE_VM ioctl. This allows
us to finalise the stage2 page table format as early as possible
and hence perform the right checks on the memory slots without
complication. The size is encoded as Log2(PA_Size) in the bits[7:0]
of the type field and can encode more information in the future if
required. The IPA size is still capped at 40bits.

Cc: Marc Zyngier <address@hidden>
Cc: Christoffer Dall <address@hidden>
Cc: Peter Maydel <address@hidden>
Cc: Paolo Bonzini <address@hidden>
Cc: Radim Krčmář <address@hidden>
Signed-off-by: Suzuki K Poulose <address@hidden>
   arch/arm/include/asm/kvm_mmu.h   |  2 ++
   arch/arm64/include/asm/kvm_arm.h | 10 +++-------
   arch/arm64/include/asm/kvm_mmu.h |  2 ++
   include/uapi/linux/kvm.h         | 10 ++++++++++
   virt/kvm/arm/arm.c               | 24 ++++++++++++++++++++++--
   5 files changed, 39 insertions(+), 9 deletions(-)


diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index 4df9bb6..fa4cab0 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -751,6 +751,16 @@ struct kvm_ppc_resize_hpt {
   #define KVM_S390_SIE_PAGE_OFFSET 1
+ * On arm/arm64, machine type can be used to request the physical
+ * address size for the VM. Bits [7-0] have been reserved for the
+ * PA size shift (i.e, log2(PA_Size)). For backward compatibility,
+ * value 0 implies the default IPA size, which is 40bits.
+ */
+#define KVM_VM_TYPE_ARM_PHYS_SHIFT(x)        \

This seems like you're allocating quite a lot of bits in a non-extensible
interface to a fairly esoteric parameter. Would it be better to add another
ioctl, or condense the number of sizes you support instead?

As I explained in the other thread, we need the size as soon as the VM
is created. The major challenge is keeping the backward compatibility by
mapping 0 to 40bits. I will give it a thought.

Here is one option. We could re-use the {V}TCR_ELx.{I}PS field format, which
occupies 3 bits and has the following definitions. (ID_AA64MMFR0_EL1:PARange
also has the field definitions, except that the field is 4bits wide, but
only 3bits are used)

000 32 bits, 4GB.
001 36 bits, 64GB.
010 40 bits, 1TB.
011 42 bits, 4TB.
100 44 bits, 16TB.
101 48 bits, 256TB.
110 52 bits, 4PB

But we need to map 0 => 40bits IPA to make our ABI backward compatible. So
we could use the additional one bit to indicate that IPA size is requested
in the 3 bits.



Bit [2:0]       - Requested IPA size. Values follow VTCR_EL2.PS format.

Bit [3]         - 1 => IPA Size bits (Bits[2:0]) requested.
                0 => Not requested

The only minor down side is restricting to the predefined values above,
which is not a real issue for a VM.

Thoughts ?

I'd be very wary of using that 4th bit to do something that is not in
the architecture. We have only a single value left to be used (0b111),
and then your scheme clashes with the architecture definition.

I agree. However, if we ever go beyond the 3bits in PARange, we have an
issue with {V}TCR counter part. But lets not take that chance.

I'd rather encode things in a way that is independent from the
architecture, and be done with it. You can map 0 to 40bits, and we have
the ability to express all values the architecture has (just in a
different order).

The other option I can think of is encoding a signed number which is the
difference of the IPA from 40. But that would need 5 bits if we were to
encode it as it is. And if we want to squeeze it in 4bit, we could store
half the difference (limiting the IPA limit to even numbers).

i.e IPA = 40 + 2 * sign_extend(bits[3:0);

I came across similar issues when trying to work out how to enable
SVE for KVM.  In the end I reduced this to a per-vcpu feature, but
it means that there is no global opt-in for the SVE-specific KVM
API extensions:

That's a bit gross, because SVE may require a change to the way
vcpus are initialised.  The set of supported SVE vector lengths needs
to be set somehow before the vcpu is set running, but it's tricky do
do that without a new ioctl -- which would mean that if SVE is enabled
for a vcpu then the vcpu is not considered runnable until the new
magic ioctl is called.

Opting into that semantic change globally at VM creation time might
be preferable.  On the SVE side, this is still very much subject to


The KVM_CREATE_VM init argument seems undefined by the KVM core code and
is available for arches to abuse in creative ways.  x86 and arm have
nothing here and reject non-zero values with -EINVAL; s390 treats it as
a bitmask, and defines a sincle feature-like bit here; powerpc treats it
as an enumeration of VM types.

If we want to be extensible, we could

  a) Pass a pointer in type, and come up with some extensible VM parameter
     struct for it to point to (which then wouldn't need a cryptic
     compressed encoding), or

  b) Introduce a new "KVM_CREATE_VM2" variant that either takes such
     an argument, or mandates a parameter negotiation phase involving
     additional ioctls before marking the VM as ready for vcpu and
     device creation.

(a) feels like an easy backwards-compatible approach, but cannot be
readily adopted by other arches (maybe not an issue).

(b) might be considered overengineered, so it would need a bit of

Wedging arguments into a few bits in the type argument feels awkward,
and may be regretted later if we run out of bits, or something can't be
represented in the chosen encoding.

I think that's a pretty convincing argument for a "better" CREATE_VM,
one that would have a clearly defined, structured (and potentially
extensible) argument.

I've quickly hacked the following:

diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index b6270a3b38e9..3e76214034c2 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -735,6 +735,20 @@ struct kvm_ppc_resize_hpt {
        __u32 pad;

+struct kvm_create_vm2 {
+       __u64   version;        /* Or maybe not */
+       union {
+               struct {
+#define KVM_ARM_SVE_CAPABLE    (1 << 0)
+#define KVM_ARM_SELECT_IPA     {1 << 1)
+                       __u64   capabilities;
+                       __u16   sve_vlen;
+                       __u8    ipa_size;
+               } arm64;
+               __u64   dummy[15];
+       };
  #define KVMIO 0xAE

  /* machine type bits, to be used as argument to KVM_CREATE_VM */

Other architectures could fill in their own bits if they need to.


This kind of thing should work, but it may still get messy when we
add additional fields.

Marc, Dave,

I like Dave's approach. Some comments below.

It we want this to work cross-arch, would it make sense to go
for a more generic approach, say

struct kvm_create_vm_attr_any {
         __u32   type;

struct kvm_create_vm_attr_arch_capabilities {
         __u32   type;
         __u16   size; /* support future expansion of capabilities[] */
         __u16   reserved;
         __u64   capabilities[1];

We also need to advertise which attributes are supported by the host,
so that the user can tune the available ones. That would make a bit mask
like the above trickier, unless we return the supported values back
in the argument ptr for the "probe" call. And this scheme in general
can be useful for passing back a non-boolean result specific to the
attribute, without having a per-attribute ioctl. (e.g, maximum limit
for IPA).

struct kvm_create_vm_attr_arm64_physaddr_size {
         __u32   type;
         __u32   physaddr_bits;

/* ... */

union kvm_create_vm_attr {
         struct kvm_create_vm_attr_any;
         struct kvm_create_vm_attr_arch_capabilities;
         struct kvm_create_vm_attr_arm64_physaddr_size;
         /* ... */

nit: Could we simply do s/kvm_create_vm_attr/kvm_vm_attr/ everywhere ?
While I agree that the kvm_create_vm_attr makes it implicit that the attributes
are valid only "create" ioctl, the lack of an ioctl to set the VM attribute
should be sufficient to indicate the same.

struct kvm_create_vm2 {
         __u32   version;        /* harmless, even if not useful */
         __u16   nr_attrs;       /* or could just terminate attrs with a
                                    NULL entry */
         union kvm_create_vm_attr __user *__user *attrs;

This is quite flexible, but obviously a bit heavy.

However, if we're adding a new interface due to lack of extensibility,
it may be worth going for something that's freely extensible.

True. I could hack something up along the lines above and send it here.

Userspace might call this as

        struct kvm_create_vm_attr_arch_capabilities vm_arch_caps = {
                .size = 64,
                .capabilities[0] = KVM_CREATE_VM_ARM64_VCPU_NEEDS_SET_SVE_VLS,

        struct kvm_create_vm_attr_arch_arm64_physaddr_size = {
                .type = KVM_CREATE_VM_ATTR_ARM64_PHYSADDR_SIZE,
                .physaddr_bits = 52,

        union kvm_create_vm_attr **vmattrs[] = {
                NULL, /* maybe */

        struct kvm_create_vm2 vm;

        vm.version = 0;
        vm.nr_attrs = 2; /* maybe */
        vm.attrs = vmattrs;

        ioctl(..., KVM_CREATE_VM2, &vm);


reply via email to

[Prev in Thread] Current Thread [Next in Thread]