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

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Date:   Wed, 04 Jul 2018 15:41:18 +0100
From:   Marc Zyngier <marc.zyngier@....com>
To:     Will Deacon <will.deacon@....com>
Cc:     Suzuki K Poulose <suzuki.poulose@....com>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>, <kvm@...r.kernel.org>,
        <kvmarm@...ts.cs.columbia.edu>, <james.morse@....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 Wed, 04 Jul 2018 15:22:42 +0100,
Will Deacon <will.deacon@....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.