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

[Zenghui Yu]

Hi Marc,

On 2019/10/29 17:23, Marc Zyngier wrote:
> On Tue, 29 Oct 2019 07:19:19 +0000,
> Zenghui Yu <yuzenghui@xxxxxxxxxx> 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@xxxxxxxxxx>
> > ---
> >
> > 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