Date:   Thu, 01 Apr 2021 14:42:21 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Eric Auger <eric.auger@...hat.com>
Cc:     eric.auger.pro@...il.com, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, kvmarm@...ts.cs.columbia.edu,
        drjones@...hat.com, alexandru.elisei@....com, james.morse@....com,
        suzuki.poulose@....com, shuah@...nel.org, pbonzini@...hat.com
Subject: Re: [PATCH v4 7/8] KVM: arm64: vgic-v3: Expose GICR_TYPER.Last for userspace

Hi Eric,

On Thu, 01 Apr 2021 09:52:37 +0100,
Eric Auger <eric.auger@...hat.com> wrote:
> 
> Commit 23bde34771f1 ("KVM: arm64: vgic-v3: Drop the
> reporting of GICR_TYPER.Last for userspace") temporarily fixed
> a bug identified when attempting to access the GICR_TYPER
> register before the redistributor region setting, but dropped
> the support of the LAST bit.
> 
> Emulating the GICR_TYPER.Last bit still makes sense for
> architecture compliance though. This patch restores its support
> (if the redistributor region was set) while keeping the code safe.
> 
> We introduce a new helper, vgic_mmio_vcpu_rdist_is_last() which
> computes whether a redistributor is the highest one of a series
> of redistributor contributor pages.
> 
> The spec says "Indicates whether this Redistributor is the
> highest-numbered Redistributor in a series of contiguous
> Redistributor pages."
> 
> The code is a bit convulated since there is no guarantee

nit: convoluted

> redistributors are added in a given reditributor region in
> ascending order. In that case the current implementation was
> wrong. Also redistributor regions can be contiguous
> and registered in non increasing base address order.
> 
> So the index of redistributors are stored in an array within
> the redistributor region structure.
> 
> With this new implementation we do not need to have a uaccess
> read accessor anymore.
> 
> Signed-off-by: Eric Auger <eric.auger@...hat.com>

This patch also hurt my head, a lot more than the first one.  See
below.

> ---
>  arch/arm64/kvm/vgic/vgic-init.c    |  7 +--
>  arch/arm64/kvm/vgic/vgic-mmio-v3.c | 97 ++++++++++++++++++++----------
>  arch/arm64/kvm/vgic/vgic.h         |  1 +
>  include/kvm/arm_vgic.h             |  3 +
>  4 files changed, 73 insertions(+), 35 deletions(-)
> 
> diff --git a/arch/arm64/kvm/vgic/vgic-init.c b/arch/arm64/kvm/vgic/vgic-init.c
> index cf6faa0aeddb2..61150c34c268c 100644
> --- a/arch/arm64/kvm/vgic/vgic-init.c
> +++ b/arch/arm64/kvm/vgic/vgic-init.c
> @@ -190,6 +190,7 @@ int kvm_vgic_vcpu_init(struct kvm_vcpu *vcpu)
>  	int i;
>  
>  	vgic_cpu->rd_iodev.base_addr = VGIC_ADDR_UNDEF;
> +	vgic_cpu->index = vcpu->vcpu_id;

Is it so that vgic_cpu->index is always equal to vcpu_id? If so, why
do we need another field? We can always get to the vcpu using a
container_of().

>  
>  	INIT_LIST_HEAD(&vgic_cpu->ap_list_head);
>  	raw_spin_lock_init(&vgic_cpu->ap_list_lock);
> @@ -338,10 +339,8 @@ static void kvm_vgic_dist_destroy(struct kvm *kvm)
>  	dist->vgic_dist_base = VGIC_ADDR_UNDEF;
>  
>  	if (dist->vgic_model == KVM_DEV_TYPE_ARM_VGIC_V3) {
> -		list_for_each_entry_safe(rdreg, next, &dist->rd_regions, list) {
> -			list_del(&rdreg->list);
> -			kfree(rdreg);
> -		}
> +		list_for_each_entry_safe(rdreg, next, &dist->rd_regions, list)
> +			vgic_v3_free_redist_region(rdreg);

Consider moving the introduction of vgic_v3_free_redist_region() into
a separate patch. On its own, that's a good readability improvement.

>  		INIT_LIST_HEAD(&dist->rd_regions);
>  	} else {
>  		dist->vgic_cpu_base = VGIC_ADDR_UNDEF;
> diff --git a/arch/arm64/kvm/vgic/vgic-mmio-v3.c b/arch/arm64/kvm/vgic/vgic-mmio-v3.c
> index 987e366c80008..f6a7eed1d6adb 100644
> --- a/arch/arm64/kvm/vgic/vgic-mmio-v3.c
> +++ b/arch/arm64/kvm/vgic/vgic-mmio-v3.c
> @@ -251,45 +251,57 @@ static void vgic_mmio_write_v3r_ctlr(struct kvm_vcpu *vcpu,
>  		vgic_enable_lpis(vcpu);
>  }
>  
> +static bool vgic_mmio_vcpu_rdist_is_last(struct kvm_vcpu *vcpu)
> +{
> +	struct vgic_dist *vgic = &vcpu->kvm->arch.vgic;
> +	struct vgic_cpu *vgic_cpu = &vcpu->arch.vgic_cpu;
> +	struct vgic_redist_region *rdreg = vgic_cpu->rdreg;
> +
> +	if (!rdreg)
> +		return false;
> +
> +	if (rdreg->count && vgic_cpu->rdreg_index == (rdreg->count - 1)) {
> +		/* check whether there is no other contiguous rdist region */
> +		struct list_head *rd_regions = &vgic->rd_regions;
> +		struct vgic_redist_region *iter;
> +
> +		list_for_each_entry(iter, rd_regions, list) {
> +			if (iter->base == rdreg->base + rdreg->count * KVM_VGIC_V3_REDIST_SIZE &&
> +				iter->free_index > 0) {
> +			/* check the first rdist index of this region, if any */
> +				if (vgic_cpu->index < iter->rdist_indices[0])
> +					return false;

rdist_indices[] contains the vcpu_id of the vcpu associated with a
given RD in the region. At this stage, you have established that there
is another region that is contiguous with the one associated with our
vcpu. You also know that this adjacent region has a vcpu mapped in
(free_index isn't 0). Isn't that enough to declare that our vcpu isn't
last?  I definitely don't understand what the index comparison does
here.

It also seem to me that some of the complexity could be eliminated if
the regions were kept ordered at list insertion time.

> +			}
> +		}
> +	} else if (vgic_cpu->rdreg_index < rdreg->free_index - 1) {
> +		/* look at the index of next rdist */
> +		int next_rdist_index = rdreg->rdist_indices[vgic_cpu->rdreg_index + 1];
> +
> +		if (vgic_cpu->index < next_rdist_index)
> +			return false;

Same thing here. We are in the middle of the allocated part of a
region, which means we cannot be last. I still don't get the index
check.

> +	}
> +	return true;
> +}
> +
>  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;
> -	struct vgic_redist_region *rdreg = vgic_cpu->rdreg;
>  	int target_vcpu_id = vcpu->vcpu_id;
> -	gpa_t last_rdist_typer = rdreg->base + GICR_TYPER +
> -			(rdreg->free_index - 1) * KVM_VGIC_V3_REDIST_SIZE;
>  	u64 value;
>  
>  	value = (u64)(mpidr & GENMASK(23, 0)) << 32;
>  	value |= ((target_vcpu_id & 0xffff) << 8);
>  
> -	if (addr == last_rdist_typer)
> +	if (vgic_has_its(vcpu->kvm))
> +		value |= GICR_TYPER_PLPIS;
> +
> +	if (vgic_mmio_vcpu_rdist_is_last(vcpu))
>  		value |= GICR_TYPER_LAST;
> -	if (vgic_has_its(vcpu->kvm))
> -		value |= GICR_TYPER_PLPIS;
>  
>  	return extract_bytes(value, addr & 7, len);
>  }
>  
> -static unsigned long vgic_uaccess_read_v3r_typer(struct kvm_vcpu *vcpu,
> -						 gpa_t addr, unsigned int len)
> -{
> -	unsigned long mpidr = kvm_vcpu_get_mpidr_aff(vcpu);
> -	int target_vcpu_id = vcpu->vcpu_id;
> -	u64 value;
> -
> -	value = (u64)(mpidr & GENMASK(23, 0)) << 32;
> -	value |= ((target_vcpu_id & 0xffff) << 8);
> -
> -	if (vgic_has_its(vcpu->kvm))
> -		value |= GICR_TYPER_PLPIS;
> -
> -	/* reporting of the Last bit is not supported for userspace */
> -	return extract_bytes(value, addr & 7, len);
> -}
> -
>  static unsigned long vgic_mmio_read_v3r_iidr(struct kvm_vcpu *vcpu,
>  					     gpa_t addr, unsigned int len)
>  {
> @@ -612,7 +624,7 @@ static const struct vgic_register_region vgic_v3_rd_registers[] = {
>  		VGIC_ACCESS_32bit),
>  	REGISTER_DESC_WITH_LENGTH_UACCESS(GICR_TYPER,
>  		vgic_mmio_read_v3r_typer, vgic_mmio_write_wi,
> -		vgic_uaccess_read_v3r_typer, vgic_mmio_uaccess_write_wi, 8,
> +		NULL, vgic_mmio_uaccess_write_wi, 8,
>  		VGIC_ACCESS_64bit | VGIC_ACCESS_32bit),
>  	REGISTER_DESC_WITH_LENGTH(GICR_WAKER,
>  		vgic_mmio_read_raz, vgic_mmio_write_wi, 4,
> @@ -714,6 +726,16 @@ int vgic_register_redist_iodev(struct kvm_vcpu *vcpu)
>  		return -EINVAL;
>  
>  	vgic_cpu->rdreg = rdreg;
> +	vgic_cpu->rdreg_index = rdreg->free_index;
> +	if (!rdreg->count) {
> +		void *p = krealloc(rdreg->rdist_indices,
> +				   (vgic_cpu->rdreg_index + 1) * sizeof(u32),
> +				   GFP_KERNEL);
> +		if (!p)
> +			return -ENOMEM;
> +		rdreg->rdist_indices = p;
> +	}
> +	rdreg->rdist_indices[vgic_cpu->rdreg_index] = vgic_cpu->index;

I think I really have a problem with this array, which comes from me
not understanding the two checks I previously commented on.

If we stick to the definition of 'Last', all that matters is the
position of the RD in a region (rdreg_index) and potentially the
presence of another contiguous region with allocated RDs in it.

IIUC, the checks should read like this:

if (vcpu->rdreg_index < (vcpu->rdreg->free_index - 1))
	last = false;
else if (vcpu->rdreg_index == (vcpu->rdreg->free_index - 1) &&
	 adjacent_region(vcpu->rdreg)->free_index > 0)
	last = false;
else
	last = true;

So why do we need to track the vcpu_id associated to a region?

>
>  	rd_base = rdreg->base + rdreg->free_index * KVM_VGIC_V3_REDIST_SIZE;
>  
> @@ -768,7 +790,7 @@ static int vgic_register_all_redist_iodevs(struct kvm *kvm)
>  }
>  
>  /**
> - * vgic_v3_insert_redist_region - Insert a new redistributor region
> + * vgic_v3_alloc_redist_region - Allocate a new redistributor region
>   *
>   * Performs various checks before inserting the rdist region in the list.
>   * Those tests depend on whether the size of the rdist region is known
> @@ -782,8 +804,8 @@ static int vgic_register_all_redist_iodevs(struct kvm *kvm)
>   *
>   * Return 0 on success, < 0 otherwise
>   */
> -static int vgic_v3_insert_redist_region(struct kvm *kvm, uint32_t index,
> -					gpa_t base, uint32_t count)
> +static int vgic_v3_alloc_redist_region(struct kvm *kvm, uint32_t index,
> +				       gpa_t base, uint32_t count)
>  {
>  	struct vgic_dist *d = &kvm->arch.vgic;
>  	struct vgic_redist_region *rdreg;
> @@ -839,6 +861,13 @@ static int vgic_v3_insert_redist_region(struct kvm *kvm, uint32_t index,
>  	rdreg->count = count;
>  	rdreg->free_index = 0;
>  	rdreg->index = index;
> +	if (count) {
> +		rdreg->rdist_indices = kcalloc(count, sizeof(u32), GFP_KERNEL);
> +		if (!rdreg->rdist_indices) {
> +			ret = -ENOMEM;
> +			goto free;
> +		}
> +	}
>  
>  	list_add_tail(&rdreg->list, rd_regions);
>  	return 0;
> @@ -847,11 +876,18 @@ static int vgic_v3_insert_redist_region(struct kvm *kvm, uint32_t index,
>  	return ret;
>  }
>  
> +void vgic_v3_free_redist_region(struct vgic_redist_region *rdreg)
> +{
> +	list_del(&rdreg->list);
> +	kfree(rdreg->rdist_indices);
> +	kfree(rdreg);
> +}
> +
>  int vgic_v3_set_redist_base(struct kvm *kvm, u32 index, u64 addr, u32 count)
>  {
>  	int ret;
>  
> -	ret = vgic_v3_insert_redist_region(kvm, index, addr, count);
> +	ret = vgic_v3_alloc_redist_region(kvm, index, addr, count);
>  	if (ret)
>  		return ret;
>  
> @@ -864,8 +900,7 @@ int vgic_v3_set_redist_base(struct kvm *kvm, u32 index, u64 addr, u32 count)
>  		struct vgic_redist_region *rdreg;
>  
>  		rdreg = vgic_v3_rdist_region_from_index(kvm, index);
> -		list_del(&rdreg->list);
> -		kfree(rdreg);
> +		vgic_v3_free_redist_region(rdreg);
>  		return ret;
>  	}
>  
> diff --git a/arch/arm64/kvm/vgic/vgic.h b/arch/arm64/kvm/vgic/vgic.h
> index 64fcd75111108..bc418c2c12141 100644
> --- a/arch/arm64/kvm/vgic/vgic.h
> +++ b/arch/arm64/kvm/vgic/vgic.h
> @@ -293,6 +293,7 @@ vgic_v3_rd_region_size(struct kvm *kvm, struct vgic_redist_region *rdreg)
>  
>  struct vgic_redist_region *vgic_v3_rdist_region_from_index(struct kvm *kvm,
>  							   u32 index);
> +void vgic_v3_free_redist_region(struct vgic_redist_region *rdreg);
>  
>  bool vgic_v3_rdist_overlap(struct kvm *kvm, gpa_t base, size_t size);
>  
> diff --git a/include/kvm/arm_vgic.h b/include/kvm/arm_vgic.h
> index 3d74f1060bd18..9a3f060ac3547 100644
> --- a/include/kvm/arm_vgic.h
> +++ b/include/kvm/arm_vgic.h
> @@ -197,6 +197,7 @@ struct vgic_redist_region {
>  	gpa_t base;
>  	u32 count; /* number of redistributors or 0 if single region */
>  	u32 free_index; /* index of the next free redistributor */
> +	int *rdist_indices; /* indices of the redistributors */

You are treating it as an array of u32 when allocating it. Please
choose one type or the other.

>  	struct list_head list;
>  };
>  
> @@ -322,6 +323,8 @@ struct vgic_cpu {
>  	 */
>  	struct vgic_io_device	rd_iodev;
>  	struct vgic_redist_region *rdreg;
> +	u32 rdreg_index;
> +	int index; /* vcpu index */
>  
>  	/* Contains the attributes and gpa of the LPI pending tables. */
>  	u64 pendbaser;

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.

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