Re: [PATCH 3/3] KVM: arm/arm64: vgic: Don't rely on the wrong pending table

[Zenghui Yu <yuzenghui@huawei.com>]

Hi Marc,

On 2019/10/29 17:23, Marc Zyngier wrote:
> On Tue, 29 Oct 2019 07:19:19 +0000,
> Zenghui Yu <yuzenghui@huawei.com> wrote:
>>
>> It's possible that two LPIs locate in the same "byte_offset" but target
>> two different vcpus, where their pending status are indicated by two
>> different pending tables.  In such a scenario, using last_byte_offset
>> optimization will lead KVM relying on the wrong pending table entry.
>> Let us use last_ptr instead, which can be treated as a byte index into
>> a pending table and also, can be vcpu specific.
>>
>> Signed-off-by: Zenghui Yu <yuzenghui@huawei.com>
>> ---
>>
>> If this patch has done the right thing, we can even add the:
>>
>> Fixes: 280771252c1b ("KVM: arm64: vgic-v3: KVM_DEV_ARM_VGIC_SAVE_PENDING_TABLES")
>>
>> But to be honest, I'm not clear about what has this patch actually fixed.
>> Pending tables should contain all zeros before we flush vgic_irq's pending
>> status into guest's RAM (thinking that guest should never write anything
>> into it). So the pending table entry we've read from the guest memory
>> seems always be zero. And we will always do the right thing even if we
>> rely on the wrong pending table entry.
>>
>> I think I must have some misunderstanding here... Please fix me.
> 
> I think you're spot on, and it is the code needs fixing, not you! The
> problem is that we only read a byte once, irrespective of the vcpu the
> interrupts is routed to. If we switch to another vcpu for the same
> byte offset, we must reload it.
> 
> This can be done by either checking the vcpu, or by tracking the guest
> address that we read from (just like you do here).

okay, the remaining question is that in vgic_v3_save_pending_tables():

	stored = val & (1U << bit_nr);
	if (stored == irq->pending_latch)
		continue;

	if (irq->pending_latch)
		val |= 1 << bit_nr;
	else
		val &= ~(1 << bit_nr);

Do we really have a scenario where irq->pending_latch==false and
stored==true (corresponds to the above "else") and then we clear
pending status of this LPI in guest memory?
I can not think out one now.

> 
> A small comment below:
> 
>>   virt/kvm/arm/vgic/vgic-v3.c | 6 +++---
>>   1 file changed, 3 insertions(+), 3 deletions(-)
>>
>> diff --git a/virt/kvm/arm/vgic/vgic-v3.c b/virt/kvm/arm/vgic/vgic-v3.c
>> index 5ef93e5041e1..7cd2e2f81513 100644
>> --- a/virt/kvm/arm/vgic/vgic-v3.c
>> +++ b/virt/kvm/arm/vgic/vgic-v3.c
>> @@ -363,8 +363,8 @@ int vgic_v3_lpi_sync_pending_status(struct kvm *kvm, struct vgic_irq *irq)
>>   int vgic_v3_save_pending_tables(struct kvm *kvm)
>>   {
>>   	struct vgic_dist *dist = &kvm->arch.vgic;
>> -	int last_byte_offset = -1;
>>   	struct vgic_irq *irq;
>> +	gpa_t last_ptr = -1;
> 
> This should be written as
> 
>       gpa_t last_ptr = ~(gpa_t)0;

Thanks for pointing it out.

> 
>>   	int ret;
>>   	u8 val;
>>   
>> @@ -384,11 +384,11 @@ int vgic_v3_save_pending_tables(struct kvm *kvm)
>>   		bit_nr = irq->intid % BITS_PER_BYTE;
>>   		ptr = pendbase + byte_offset;
>>   
>> -		if (byte_offset != last_byte_offset) {
>> +		if (ptr != last_ptr) {
>>   			ret = kvm_read_guest_lock(kvm, ptr, &val, 1);
>>   			if (ret)
>>   				return ret;
>> -			last_byte_offset = byte_offset;
>> +			last_ptr = ptr;
>>   		}
>>   
>>   		stored = val & (1U << bit_nr);
> 
> Otherwise, this looks good to me (no need to respin for the above
> nit).

Thanks,

Zenghui


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