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

[Andre Przywara <andre.przywara@arm.com>]

On Sun, 22 Aug 2021 16:25:26 +0100
Marc Zyngier <maz@kernel.org> wrote:

Hi Marc,

> 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.

You mentioned some kernel bug in the first version of this patch, I
guess the fix for this is 262b003d059c?
It seems to me that this is somewhat of a regression on older host
kernels, when trying to run a guest with "-m 2048", for instance?
Should we teach kvmtool about this bug, and do a check for the bug
condition, when kvm__register_ram() returns with -EFAULT? And give users
a hint to try with one MB more or less guest RAM? Or maybe try
this automatically?

> 
> 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;

Should this return KVM_VM_TYPE, as it does at the moment? Or is this
more confusing than helpful?

Just a nit anyway, the patch looks correct otherwise:

Reviewed-by: Andre Przywara <andre.przywara@arm.com>

Cheers,
Andre

> +
> +	/* 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);
> +
> +	return KVM_VM_TYPE_ARM_IPA_SIZE(ipa_bits);
>  }

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