Re: [PATCH v2 3/3] kvmtool: arm64: Configure VM with the minimal required IPA space

[Oliver Upton <oupton@google.com>]

Marc,

On Sun, Aug 22, 2021 at 8:25 AM Marc Zyngier <maz@kernel.org> wrote:
>
> There is some value in keeping the IPA space small, as it reduces
> the size of the stage-2 page tables.
>
> Let's compute the required space at VM creation time, and inform
> the kernel of our requirements.
>
> Signed-off-by: Marc Zyngier <maz@kernel.org>
> ---
>  arm/aarch64/kvm.c | 20 +++++++++++++++++++-
>  1 file changed, 19 insertions(+), 1 deletion(-)
>
> diff --git a/arm/aarch64/kvm.c b/arm/aarch64/kvm.c
> index d03a27f2..4e66a22e 100644
> --- a/arm/aarch64/kvm.c
> +++ b/arm/aarch64/kvm.c
> @@ -3,6 +3,7 @@
>  #include <asm/image.h>
>
>  #include <linux/byteorder.h>
> +#include <kvm/util.h>
>
>  /*
>   * Return the TEXT_OFFSET value that the guest kernel expects. Note
> @@ -59,5 +60,22 @@ int kvm__arch_get_ipa_limit(struct kvm *kvm)
>
>  int kvm__get_vm_type(struct kvm *kvm)
>  {
> -       return KVM_VM_TYPE_ARM_IPA_SIZE(kvm__arch_get_ipa_limit(kvm));
> +       unsigned int ipa_bits, max_ipa_bits;
> +       unsigned long max_ipa;
> +
> +       /* If we're running on an old kernel, use 0 as the VM type */
> +       max_ipa_bits = kvm__arch_get_ipa_limit(kvm);
> +       if (!max_ipa_bits)
> +               return 0;
> +
> +       /* Otherwise, compute the minimal required IPA size */
> +       max_ipa = ARM_MEMORY_AREA + kvm->cfg.ram_size - 1;
> +       ipa_bits = max(32, fls_long(max_ipa));
> +       pr_debug("max_ipa %lx ipa_bits %d max_ipa_bits %d",
> +                max_ipa, ipa_bits, max_ipa_bits);
> +
> +       if (ipa_bits > max_ipa_bits)
> +               die("Memory too large for this system (needs %d bits, %d available)", ipa_bits, max_ipa_bits);

I imagine it may not be immediately obvious to a kvmtool user what
this log line means, like what 'bits' are being referred to here.
Would it be more helpful to report the maximum allowed memory size for
the system, as derived from the max IPA?

--
Thanks,
Oliver

> +
> +       return KVM_VM_TYPE_ARM_IPA_SIZE(ipa_bits);
>  }
> --
> 2.30.2
>
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