Re: [PATCH v5 7/8] KVM: arm64: vgic-v3: Expose GICR_TYPER.Last for userspace

Marc Zyngier
2021-04-05T10:10:23Z

On Sun, 04 Apr 2021 18:22:42 +0100,
Eric Auger <eric.auger@redhat.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@redhat.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.