Re: [PATCH v2] accel/kvm: Specify default IPA size for arm64

[Peter Maydell <peter.maydell@linaro.org>]

On Fri, 21 Jul 2023 at 08:30, Philippe Mathieu-Daudé <philmd@linaro.org> wrote:
>
> Hi Akihiko,
>
> On 21/7/23 08:24, Akihiko Odaki wrote:
> > libvirt uses "none" machine type to test KVM availability. Before this
> > change, QEMU used to pass 0 as machine type when calling KVM_CREATE_VM.
> >
> > 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, such KVM_CREATE_VM will fail, and libvirt
> > incorrectly thinks KVM is not available. This actually happened on M2
> > MacBook Air.
> >
> > Fix this by specifying 32 for IPA_Bits as any arm64 system should
> > support the value according to the documentation.
> >
> > Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
> > ---
> > V1 -> V2: Introduced an arch hook
> >
> >   include/sysemu/kvm.h   | 1 +
> >   accel/kvm/kvm-all.c    | 2 +-
> >   target/arm/kvm.c       | 2 ++
> >   target/i386/kvm/kvm.c  | 2 ++
> >   target/mips/kvm.c      | 2 ++
> >   target/ppc/kvm.c       | 2 ++
> >   target/riscv/kvm.c     | 2 ++
> >   target/s390x/kvm/kvm.c | 2 ++
> >   8 files changed, 14 insertions(+), 1 deletion(-)
>
> My understanding of Peter's suggestion would be smth like:
>
> -- >8 --
> diff --git a/include/sysemu/kvm.h b/include/sysemu/kvm.h
> index 115f0cca79..c0af15eb6c 100644
> --- a/include/sysemu/kvm.h
> +++ b/include/sysemu/kvm.h
> @@ -201,10 +201,15 @@ typedef struct KVMCapabilityInfo {
>
>   struct KVMState;
>
> +struct KVMClass {
> +    AccelClass parent_class;
> +
> +    int default_vm_type;

The kernel docs say you need to check for the
KVM_CAP_ARM_VM_IPA_SIZE before you can pass something other
than zero to the KVM_CREATE_VM ioctl, so this needs to be
a method, not just a value. (kvm_arm_get_max_vm_ipa_size()
will do this bit for you.)

If the machine doesn't provide a kvm_type method, we
should default to "largest the host supports", I think.

I was wondering if we could have one per-arch
method for "actually create the VM" that both was
a place for arm to set the default vm type and
also let us get the TARGET_S390X and TARGET_PPC
ifdefs out of this bit of kvm-all.c, but maybe that would
look just a bit too awkward:

     if (kc->create_vm(s, board_sets_kvm_type, board_kvm_type) < 0) {
         goto err;
     }

where board_sets_kvm_type is a bool, true if board_kvm_type
is valid, and board_kvm_type is whatever the board's
mc->kvm_type method told us.

(Default impl of the method: call KVM_CREATE_VM ioctl
with retry-on-eintr, printing the simple error message;
PPC and s390 versions similar but with their arch
specific extra messages; arm version has a different
default type if board_sets_kvm_type is false.)

Not trying to do both of those things with one method
would result in a simpler
   type = kc->get_default_kvm_type(s);
API.

thanks
-- PMM