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

On 2020/3/31 16:07, Marc Zyngier wrote:
> 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.

A writing to GICR_CTLR.EnableLPIs can happen in parallel with MAPTI/INT
command sequence, where the new LPI is mapped to *this* vcpu and made
pending, wrong? I think commit 7d8b44c54e0c had described it in detail.

But thinking that we cache the pending bit in pending_latch (instead of
writing the corresponding bit in guest memory) when a LPI is made
pending, it seems to be safe to clear the whole pending table here.

>
> 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?

vgic_v3_lpi_sync_pending_status() can be called on the ITE restore path:
vgic_its_restore_ite/vgic_add_lpi/vgic_v3_lpi_sync_pending_status.
We should rely on it to sync the pending bit from guest memory (which
was saved on the source side).

> 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").

;-)


Thanks,
Zenghui

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