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SubjectRe: [kvmtool test PATCH 24/24] kvmtool: arm: Add support for creating VM with PA size
On Wed, 04 Jul 2018 15:22:42 +0100,
Will Deacon <will.deacon@arm.com> wrote:
>
> 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?

Not really. Let's say I want my IPA space split in two: memory covers
the low 47 bit, and I want MMIO spanning the top 47 bit. With your
scheme, you'd end-up with a 47bit IPA space, while you really want 48
bits (MMIO space implemented by userspace isn't registered to the
kernel).

M.

--
Jazz is not dead, it just smell funny.

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