linux-kernel - Re: [PATCH v5 7/8] KVM: arm64: vgic-v3: Expose GICR

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Date:   Mon, 05 Apr 2021 11:10:18 +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 v5 7/8] KVM: arm64: vgic-v3: Expose GICR_TYPER.Last for userspace

On Sun, 04 Apr 2021 18:22:42 +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.
> 
> With this new implementation we do not need to have a uaccess
> read accessor anymore.
> 
> Signed-off-by: Eric Auger <eric.auger@...hat.com>
> 
> ---
> 
> v4 -> v5:
> - redist region list now is sorted by @base
> - change the implementation according to Marc's understanding of
>   the spec
> ---
>  arch/arm64/kvm/vgic/vgic-mmio-v3.c | 58 +++++++++++++++++-------------
>  include/kvm/arm_vgic.h             |  1 +
>  2 files changed, 34 insertions(+), 25 deletions(-)
> 
> diff --git a/arch/arm64/kvm/vgic/vgic-mmio-v3.c b/arch/arm64/kvm/vgic/vgic-mmio-v3.c
> index e1ed0c5a8eaa..03a253785700 100644
> --- a/arch/arm64/kvm/vgic/vgic-mmio-v3.c
> +++ b/arch/arm64/kvm/vgic/vgic-mmio-v3.c
> @@ -251,45 +251,52 @@ 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 *iter, *rdreg = vgic_cpu->rdreg;
> +
> +	if (!rdreg)
> +		return false;
> +
> +	if (vgic_cpu->rdreg_index < rdreg->free_index - 1) {
> +		return false;
> +	} else if (rdreg->count && vgic_cpu->rdreg_index == (rdreg->count - 1)) {
> +		struct list_head *rd_regions = &vgic->rd_regions;
> +		gpa_t end = rdreg->base + rdreg->count * KVM_VGIC_V3_REDIST_SIZE;
> +
> +		/*
> +		 * the rdist is the last one of the redist region,
> +		 * check whether there is no other contiguous rdist region
> +		 */
> +		list_for_each_entry(iter, rd_regions, list) {
> +			if (iter->base == end && iter->free_index > 0)
> +				return false;
> +		}

In the above notes, you state that the region list is now sorted by
base address, but I really can't see what sorts that list. And the
lines above indicate that you are still iterating over the whole RD
regions.

It's not a big deal (the code is now simple enough), but that's just
to confirm that I understand what is going on here.

Thanks,

	M.

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