On Tue, 26 Apr 2022 17:21:07 +0100,
Ricardo Koller <ricarkol@xxxxxxxxxx> wrote:
>
> On Tue, Apr 26, 2022 at 05:07:40AM +0100, Marc Zyngier wrote:
> > On Mon, 25 Apr 2022 19:55:31 +0100,
> > Ricardo Koller <ricarkol@xxxxxxxxxx> wrote:
> > >
> > > A command that adds an entry into an ITS table that is not in guest
> > > memory should fail, as any command should be treated as if it was
> > > actually saving entries in guest memory (KVM doesn't until saving).
> > > Add the corresponding check for the ITT when adding ITEs.
> > >
> > > Signed-off-by: Ricardo Koller <ricarkol@xxxxxxxxxx>
> > > ---
> > > arch/arm64/kvm/vgic/vgic-its.c | 34 ++++++++++++++++++++++++++++++++++
> > > 1 file changed, 34 insertions(+)
> > >
> > > diff --git a/arch/arm64/kvm/vgic/vgic-its.c b/arch/arm64/kvm/vgic/vgic-its.c
> > > index 2e13402be3bd..d7c1a3a01af4 100644
> > > --- a/arch/arm64/kvm/vgic/vgic-its.c
> > > +++ b/arch/arm64/kvm/vgic/vgic-its.c
> > > @@ -976,6 +976,37 @@ static bool vgic_its_check_id(struct vgic_its *its, u64 baser, u32 id,
> > > return ret;
> > > }
> > >
> > > +/*
> > > + * Check whether an event ID can be stored in the corresponding Interrupt
> > > + * Translation Table, which starts at device->itt_addr.
> > > + */
> > > +static bool vgic_its_check_ite(struct vgic_its *its, struct its_device *device,
> > > + u32 event_id)
> > > +{
> > > + const struct vgic_its_abi *abi = vgic_its_get_abi(its);
> > > + int ite_esz = abi->ite_esz;
> > > + gpa_t gpa;
> > > + gfn_t gfn;
> > > + int idx;
> > > + bool ret;
> > > +
> > > + /* max table size is: BIT_ULL(device->num_eventid_bits) * ite_esz */
> > > + if (event_id >= BIT_ULL(device->num_eventid_bits))
> > > + return false;
> >
> > We have already checked this condition, it seems.
> >
> > > +
> > > + gpa = device->itt_addr + event_id * ite_esz;
> > > + gfn = gpa >> PAGE_SHIFT;
> > > +
> > > + idx = srcu_read_lock(&its->dev->kvm->srcu);
> > > + ret = kvm_is_visible_gfn(its->dev->kvm, gfn);
> > > + srcu_read_unlock(&its->dev->kvm->srcu, idx);
> > > + return ret;
> >
> > Why should we care? If the guest doesn't give us the memory that is
> > required, that's its problem.
>
> The issue is that if the guest does that, then the pause will fail and
> we won't be able to migrate the VM. This series objective is to help
> with failed migrations due to the ITS. This commit tries to do it by
> avoiding them.
But the guest is buggy, isn't it? No memory, no cookie! ;-)
I understand that you want save/restore to be predictable even when
the guest is too crap for words. I think clarifying this in your
commit message would help.
> > The only architectural requirement is
> > that the EID fits into the device table. There is no guarantee that
> > the ITS will actually write to the memory.
>
> If I understand it correctly, failing the command in this case would
> also be architectural (right?). If the ITS tries to write the new
> entry into memory immediately, then the command would fail. This
> proposed check is just making this assumption.
Neither behaviour is architectural (they are both allowed, but none
is required). Not providing the memory, however, is unpredictable.
I'm OK with your approach because it makes things consistent (we fail
early rather than late). But the commit message doesn't really reflect
that (it sort of hints to it, but not in a clear way).
Thanks,
M.
--
Without deviation from the norm, progress is not possible.
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