linux-kernel - Re: [RFC 03/12] KVM: arm/arm64: Record RDIST Last bit at registration

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Date:   Mon, 19 Mar 2018 15:57:21 +0000
From:   Marc Zyngier <marc.zyngier@....com>
To:     Eric Auger <eric.auger@...hat.com>, eric.auger.pro@...il.com,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        kvmarm@...ts.cs.columbia.edu, cdall@...nel.org,
        peter.maydell@...aro.org
Cc:     andre.przywara@....com, drjones@...hat.com, wei@...hat.com
Subject: Re: [RFC 03/12] KVM: arm/arm64: Record RDIST Last bit at registration

On 19/03/18 09:20, Eric Auger wrote:
> To prepare for multiple RDIST regions, let's record the RDIST
> Last bit field when registering the IO device.
> 
> As a reminder the Last bit indicates whether the redistributor
> is the highest one in a series of contiguous redistributor pages.
> 
> Since at the moment KVM only supports a single redist region,
> the RDIST tagged with the last bit set to true corresponds to the
> highest vCPU one.
> 
> Signed-off-by: Eric Auger <eric.auger@...hat.com>
> ---
>  include/kvm/arm_vgic.h           | 1 +
>  virt/kvm/arm/vgic/vgic-mmio-v3.c | 6 +++++-
>  2 files changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/include/kvm/arm_vgic.h b/include/kvm/arm_vgic.h
> index cdbd142..1c8c0ff 100644
> --- a/include/kvm/arm_vgic.h
> +++ b/include/kvm/arm_vgic.h
> @@ -312,6 +312,7 @@ struct vgic_cpu {
>  	 */
>  	struct vgic_io_device	rd_iodev;
>  	struct vgic_io_device	sgi_iodev;
> +	bool rdist_last; /* Is the RDIST the last one of the RDIST region? */
>  
>  	/* Contains the attributes and gpa of the LPI pending tables. */
>  	u64 pendbaser;
> diff --git a/virt/kvm/arm/vgic/vgic-mmio-v3.c b/virt/kvm/arm/vgic/vgic-mmio-v3.c
> index 671fe81..75fe689 100644
> --- a/virt/kvm/arm/vgic/vgic-mmio-v3.c
> +++ b/virt/kvm/arm/vgic/vgic-mmio-v3.c
> @@ -184,12 +184,13 @@ static unsigned long vgic_mmio_read_v3r_typer(struct kvm_vcpu *vcpu,
>  					      gpa_t addr, unsigned int len)
>  {
>  	unsigned long mpidr = kvm_vcpu_get_mpidr_aff(vcpu);
> +	struct vgic_cpu *vgic_cpu = &vcpu->arch.vgic_cpu;
>  	int target_vcpu_id = vcpu->vcpu_id;
>  	u64 value;
>  
>  	value = (u64)(mpidr & GENMASK(23, 0)) << 32;
>  	value |= ((target_vcpu_id & 0xffff) << 8);
> -	if (target_vcpu_id == atomic_read(&vcpu->kvm->online_vcpus) - 1)
> +	if (vgic_cpu->rdist_last)
>  		value |= GICR_TYPER_LAST;
>  	if (vgic_has_its(vcpu->kvm))
>  		value |= GICR_TYPER_PLPIS;
> @@ -580,6 +581,7 @@ int vgic_register_redist_iodev(struct kvm_vcpu *vcpu)
>  {
>  	struct kvm *kvm = vcpu->kvm;
>  	struct vgic_dist *vgic = &kvm->arch.vgic;
> +	struct vgic_cpu *vgic_cpu = &vcpu->arch.vgic_cpu;
>  	struct vgic_io_device *rd_dev = &vcpu->arch.vgic_cpu.rd_iodev;
>  	struct vgic_io_device *sgi_dev = &vcpu->arch.vgic_cpu.sgi_iodev;
>  	gpa_t rd_base, sgi_base;
> @@ -632,6 +634,8 @@ int vgic_register_redist_iodev(struct kvm_vcpu *vcpu)
>  	}
>  
>  	vgic->vgic_redist_free_offset += 2 * SZ_64K;
> +	vgic_cpu->rdist_last =
> +		(vcpu->vcpu_id == atomic_read(&vcpu->kvm->online_vcpus) - 1);
>  out:
>  	mutex_unlock(&kvm->slots_lock);
>  	return ret;
> 

I don't really like the idea of keeping this "Last" bit around, because
it doesn't have much to do with the state of a redistributor, and has
more to do with its position in the region.

What is wrong with the current approach of dynamically computing the
Last bit? If you have multiple regions, all you need to know is which
region this redistributor belongs to. From that point (and assuming you
know how many redistributors have been instantiated in that region, you
can know whether the Last bit is set or not.

One of the issue is that the current code works in term of "target cpu",
while we really want is to use the addr parameter to work out if the
Last bit is set or not. I'd be a lot more confident if we emulate it
like that (which is how the HW would generate it too).

What do you think?

	M.
-- 
Jazz is not dead. It just smells funny...