Before this change, the default KVM type, which is used for non-virt
machine models, was 0.

The kernel documentation says:
> 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 size, 40bits (for backward compatibility)
>   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.
>
> Creation of the VM will fail if the requested IPA size (whether it is
> implicit or explicit) is unsupported on the host.
https://docs.kernel.org/virt/kvm/api.html#kvm-create-vm

So if Host_IPA_Limit < 40, specifying 0 as the type will fail. This
actually confused libvirt, which uses "none" machine model to probe the
KVM availability, on M2 MacBook Air.

Fix this by using Host_IPA_Limit as the default type when
KVM_CAP_ARM_VM_IPA_SIZE is available.

Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
---
 target/arm/kvm.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/target/arm/kvm.c b/target/arm/kvm.c
index 40f577bfd5..23aeb09949 100644
--- a/target/arm/kvm.c
+++ b/target/arm/kvm.c
@@ -249,7 +249,9 @@ int kvm_arm_get_max_vm_ipa_size(MachineState *ms, bool *fixed_ipa)
 
 int kvm_arch_get_default_type(MachineState *ms)
 {
-    return 0;
+    bool fixed_ipa;
+    int size = kvm_arm_get_max_vm_ipa_size(ms, &fixed_ipa);
+    return fixed_ipa ? 0 : size;
 }
 
 int kvm_arch_init(MachineState *ms, KVMState *s)
-- 
2.41.0

On Thu, 27 Jul 2023 at 08:31, Akihiko Odaki <akihiko.odaki@daynix.com> wrote:
>
> Before this change, the default KVM type, which is used for non-virt
> machine models, was 0.
>
> The kernel documentation says:
> > 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 size, 40bits (for backward compatibility)
> >   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.
> >
> > Creation of the VM will fail if the requested IPA size (whether it is
> > implicit or explicit) is unsupported on the host.
> https://docs.kernel.org/virt/kvm/api.html#kvm-create-vm
>
> So if Host_IPA_Limit < 40, specifying 0 as the type will fail. This
> actually confused libvirt, which uses "none" machine model to probe the
> KVM availability, on M2 MacBook Air.
>
> Fix this by using Host_IPA_Limit as the default type when
> KVM_CAP_ARM_VM_IPA_SIZE is available.
>
> Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>

thanks
-- PMM