linux-kernel - Re: [kvmtool test PATCH 24/24] kvmtool: arm: Add support for creating VM with PA size

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Date:   Wed, 4 Jul 2018 15:22:42 +0100
From:   Will Deacon <will.deacon@....com>
To:     Suzuki K Poulose <suzuki.poulose@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, kvmarm@...ts.cs.columbia.edu,
        james.morse@....com, marc.zyngier@....com, cdall@...nel.org,
        eric.auger@...hat.com, julien.grall@....com,
        catalin.marinas@....com, punit.agrawal@....com,
        qemu-devel@...gnu.org
Subject: Re: [kvmtool test PATCH 24/24] kvmtool: arm: Add support for
 creating VM with PA size

On Fri, Jun 29, 2018 at 12:15:44PM +0100, Suzuki K Poulose wrote:
> diff --git a/arm/kvm.c b/arm/kvm.c
> index 5701d41..b1969be 100644
> --- a/arm/kvm.c
> +++ b/arm/kvm.c
> @@ -11,6 +11,8 @@
>  #include <linux/kvm.h>
>  #include <linux/sizes.h>
>  
> +unsigned long kvm_arm_type;
> +
>  struct kvm_ext kvm_req_ext[] = {
>  	{ DEFINE_KVM_EXT(KVM_CAP_IRQCHIP) },
>  	{ DEFINE_KVM_EXT(KVM_CAP_ONE_REG) },
> @@ -18,6 +20,26 @@ struct kvm_ext kvm_req_ext[] = {
>  	{ 0, 0 },
>  };
>  
> +#ifndef KVM_ARM_GET_MAX_VM_PHYS_SHIFT
> +#define KVM_ARM_GET_MAX_VM_PHYS_SHIFT		_IO(KVMIO, 0x0b)
> +#endif
> +
> +void kvm__arch_init_hyp(struct kvm *kvm)
> +{
> +	int max_ipa;
> +
> +	max_ipa = ioctl(kvm->sys_fd, KVM_ARM_GET_MAX_VM_PHYS_SHIFT);
> +	if (max_ipa < 0)
> +		max_ipa = 40;
> +	if (!kvm->cfg.arch.phys_shift)
> +		kvm->cfg.arch.phys_shift = 40;
> +	if (kvm->cfg.arch.phys_shift > max_ipa)
> +		die("Requested PA size (%u) is not supported by the host (%ubits)\n",
> +		    kvm->cfg.arch.phys_shift, max_ipa);
> +	if (kvm->cfg.arch.phys_shift != 40)
> +		kvm_arm_type = kvm->cfg.arch.phys_shift;
> +}

Seems a bit weird that the "machine type identifier" to KVM_CREATE_VM is
dedicated entirely to holding the physical address shift verbatim. Is this
really the ABI?

Also, couldn't KVM figure it out automatically if you add memslots at high
addresses, making this a niche tunable outside of testing?

Will