Re: [PATCH] KVM: arm/arm64: vgic: Handle GICR_PENDBASER.PTZ filed as RAZ

[Zenghui Yu]

On 2019/12/24 12:45, Auger Eric wrote:
> Hi Zenghui,
>
> On 12/24/19 3:52 AM, Zenghui Yu wrote:
> > Hi Marc, Eric,
> >
> > On 2019/12/23 22:07, Marc Zyngier wrote:
> > > Hi Zenghui,
> > >
> > > On 2019-12-23 13:43, Zenghui Yu wrote:
> > > > I noticed there is no userspace access callbacks for GICR_PENDBASER,
> > > > so this patch will make the PTZ field also 'Read As Zero' by userspace.
> > > > Should we consider adding a uaccess_read callback for GICR_PENDBASER
> > > > which just returns the unchanged vgic_cpu->pendbaser to userspace?
> > > > (Though this is really not a big deal. We now always emulate the PTZ
> > > > field to guest as RAZ. And 'vgic_cpu->pendbaser & GICR_PENDBASER_PTZ'
> > > > only indicates whether KVM will optimize the LPI enabling process,
> > > > where Read As Zero indicates never optimize..)
> > >
> > > I don't think adding a userspace accessor would help much. All this
> > > bit tells userspace is that the guest has programmed a zero filled
> > > table. On restore, we'd avoid a rescan of the table if there was
> > > no LPI mapped.
> >
> > Yes, I agree.
> >
> > > And thinking of it, this fixes a bug for non-Linux guests: If you write
> > > PTZ=1, we never clear it. Which means that if userspace saves and
> > > restores
> > > PENDBASER with PTZ set, we'll never restore the pending bits, which is
> > > pretty bad (see vgic_enable_lpis()).
> >
> > But I'm afraid I can't follow this point. After reading the code (with
> > Qemu) a bit further, the Redistributors are restored before the ITS.
>
> This is also part of the kernel documentation:
> Documentation/virt/kvm/devices/arm-vgic-its.txt (ITS restore sequence)

Yeah, I see. Thanks for the pointer, Eric!


Zenghui

>   So
> > there should be _no_ LPI has been mapped when we're restoring GICR_CTLR
> > and enabling LPI, which says we will not scan the whole pending table
> > and restore pending by vgic_enable_lpis()/its_sync_lpi_pending_table(),
> > regardless of what the PTZ is.
> >
> > Instead, vgic_its_restore_ite()/vgic_v3_lpi_sync_pending_status() is
> > where we actually read the guest RAM and restore the LPI pending state.
> yes the pending state is restored from
> vgic_its_restore_ite/vgic_add_lpi/vgic_v3_lpi_sync_pending_status and
> this path ignores the PTZ.
>
> Thanks
>
> Eric
> > Which means we will still do the right thing even for non-Linux guests.
> > Not sure if I've got things correctly here.
> >
> > In the end, let's keep the patch as it is.
> >
> > > This patch on its own fixes more than one bug!
> >
> > If so, just by luck ;-)