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SubjectRe: [PATCH] KVM: arm64: vgic-v3: Clear pending bit in guest memory after synchronization
Hi Zenghui,

On Tue, 31 Mar 2020 11:12:45 +0800
Zenghui Yu <yuzenghui@huawei.com> wrote:

> When LPI support is enabled at redistributor level, VGIC will potentially
> load the correspond LPI penging table and sync it into the pending_latch.
> To avoid keeping the 'consumed' pending bits lying around in guest memory
> (though they're not used), let's clear them after synchronization.
>
> The similar work had been done in vgic_v3_lpi_sync_pending_status().
>
> Signed-off-by: Zenghui Yu <yuzenghui@huawei.com>
> ---
> virt/kvm/arm/vgic/vgic-its.c | 23 +++++++++++++++++------
> 1 file changed, 17 insertions(+), 6 deletions(-)
>
> diff --git a/virt/kvm/arm/vgic/vgic-its.c b/virt/kvm/arm/vgic/vgic-its.c
> index d53d34a33e35..905760bfa404 100644
> --- a/virt/kvm/arm/vgic/vgic-its.c
> +++ b/virt/kvm/arm/vgic/vgic-its.c
> @@ -435,6 +435,7 @@ static int its_sync_lpi_pending_table(struct kvm_vcpu *vcpu)
>
> for (i = 0; i < nr_irqs; i++) {
> int byte_offset, bit_nr;
> + bool status;
>
> byte_offset = intids[i] / BITS_PER_BYTE;
> bit_nr = intids[i] % BITS_PER_BYTE;
> @@ -447,22 +448,32 @@ static int its_sync_lpi_pending_table(struct kvm_vcpu *vcpu)
> ret = kvm_read_guest_lock(vcpu->kvm,
> pendbase + byte_offset,
> &pendmask, 1);
> - if (ret) {
> - kfree(intids);
> - return ret;
> - }
> + if (ret)
> + goto out;
> last_byte_offset = byte_offset;
> }
>
> + status = pendmask & (1 << bit_nr);
> +
> irq = vgic_get_irq(vcpu->kvm, NULL, intids[i]);
> raw_spin_lock_irqsave(&irq->irq_lock, flags);
> - irq->pending_latch = pendmask & (1U << bit_nr);
> + irq->pending_latch = status;
> vgic_queue_irq_unlock(vcpu->kvm, irq, flags);
> vgic_put_irq(vcpu->kvm, irq);
> +
> + if (status) {
> + /* clear consumed data */
> + pendmask &= ~(1 << bit_nr);
> + ret = kvm_write_guest_lock(vcpu->kvm,
> + pendbase + byte_offset,
> + &pendmask, 1);
> + if (ret)
> + goto out;
> + }
> }
>
> +out:
> kfree(intids);
> -
> return ret;
> }
>

I've been thinking about this, and I wonder why we don't simply clear
the whole pending table instead of carefully wiping it one bit at a
time. My reasoning is that if a LPI isn't mapped, then it cannot be made
pending the first place.

And I think there is a similar issue in vgic_v3_lpi_sync_pending_status().
Why sync something back from the pending table when the LPI wasn't
mapped yet? This seems pretty bizarre, as the GITS_TRANSLATER spec says
that the write to this register is ignored when:

"- The EventID is mapped to an Interrupt Translation Table and the
EventID is within the range specified by MAPD on page 5-107, but the
EventID is unmapped."

(with the added bonus in the form of a type: the first instance of
"EventID" here should obviously be "DeviceID").

What do you think?

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

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