linux-kernel - Re: [PATCH] KVM: arm64: vgic-v3: Clear pending bit in guest memory after synchronization

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Date:   Tue, 31 Mar 2020 17:11:37 +0800
From:   Zenghui Yu <yuzenghui@...wei.com>
To:     Marc Zyngier <maz@...nel.org>
CC:     <kvmarm@...ts.cs.columbia.edu>, <eric.auger@...hat.com>,
        <andre.przywara@....com>, <james.morse@....com>,
        <julien.thierry.kdev@...il.com>, <suzuki.poulose@....com>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>, <wanghaibin.wang@...wei.com>
Subject: Re: [PATCH] KVM: arm64: vgic-v3: Clear pending bit in guest memory
 after synchronization

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@...wei.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@...wei.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