Re: [PATCH v5 01/22] KVM: arm/arm64: Add ITS save/restore API documentation

Christoffer Dall
2017-04-27T17:27:04Z

On Thu, Apr 27, 2017 at 07:14:29PM +0200, Auger Eric wrote:
> Hi Marc, Christoffer,
>
> On 27/04/2017 18:23, Marc Zyngier wrote:
> > On 27/04/17 16:29, Auger Eric wrote:
> >>
> >>
> >> On 27/04/2017 16:45, Christoffer Dall wrote:
> >>> Hi Eric,
> >>>
> >>> On Thu, Apr 27, 2017 at 02:51:00PM +0200, Auger Eric wrote:
> >>>> On 27/04/2017 13:02, Christoffer Dall wrote:
> >>>>> On Thu, Apr 27, 2017 at 11:33:39AM +0200, Auger Eric wrote:
> >>>>>> On 27/04/2017 10:57, Christoffer Dall wrote:
> >>>>>>> On Wed, Apr 26, 2017 at 05:48:32PM +0200, Auger Eric wrote:
> >>>>>>>> On 26/04/2017 14:31, Christoffer Dall wrote:
> >>>>>>>>> On Fri, Apr 14, 2017 at 12:15:13PM +0200, Eric Auger wrote:
> >>>>>>>>>> Add description for how to access ITS registers and how to save/restore
> >>>>>>>>>> ITS tables into/from memory.
> >>>>>>>>>>
> >>>>>>>>>> Signed-off-by: Eric Auger <eric.auger@redhat.com>
> >>>>>>>>>>
> >>>>>>>>>> ---
> >>>>>>>>>> v4 -> v5:
> >>>>>>>>>> - take into account Christoffer's comments
> >>>>>>>>>> - pending table save on GICV3 side now
> >>>>>>>>>>
> >>>>>>>>>> v3 -> v4:
> >>>>>>>>>> - take into account Peter's comments:
> >>>>>>>>>> - typos
> >>>>>>>>>> - KVM_DEV_ARM_VGIC_GRP_ITS_TABLES kvm_device_attr = 0
> >>>>>>>>>> - add a validity bit in DTE
> >>>>>>>>>> - document all fields in CTE and ITE
> >>>>>>>>>> - document ABI revision
> >>>>>>>>>> - take into account Andre's comments:
> >>>>>>>>>> - document restrictions about GITS_CREADR writing and GITS_IIDR
> >>>>>>>>>> - document -EBUSY error if one or more VCPUS are runnning
> >>>>>>>>>> - document 64b registers only can be accessed with 64b access
> >>>>>>>>>> - itt_addr field matches bits [51:8] of the itt_addr
> >>>>>>>>>>
> >>>>>>>>>> v1 -> v2:
> >>>>>>>>>> - DTE and ITE now are 8 bytes
> >>>>>>>>>> - DTE and ITE now indexed by deviceid/eventid
> >>>>>>>>>> - use ITE name instead of ITTE
> >>>>>>>>>> - mentions ITT_addr matches bits [51:8] of the actual address
> >>>>>>>>>> - mentions LE layout
> >>>>>>>>>> ---
> >>>>>>>>>> Documentation/virtual/kvm/devices/arm-vgic-its.txt | 99 ++++++++++++++++++++++
> >>>>>>>>>> 1 file changed, 99 insertions(+)
> >>>>>>>>>>
> >>>>>>>>>> diff --git a/Documentation/virtual/kvm/devices/arm-vgic-its.txt b/Documentation/virtual/kvm/devices/arm-vgic-its.txt
> >>>>>>>>>> index 6081a5b..b5f010d 100644
> >>>>>>>>>> --- a/Documentation/virtual/kvm/devices/arm-vgic-its.txt
> >>>>>>>>>> +++ b/Documentation/virtual/kvm/devices/arm-vgic-its.txt
> >>>>>>>>>> @@ -32,7 +32,106 @@ Groups:
> >>>>>>>>>> KVM_DEV_ARM_VGIC_CTRL_INIT
> >>>>>>>>>> request the initialization of the ITS, no additional parameter in
> >>>>>>>>>> kvm_device_attr.addr.
> >>>>>>>>>> +
> >>>>>>>>>> + KVM_DEV_ARM_ITS_SAVE_TABLES
> >>>>>>>>>> + save the ITS table data into guest RAM, at the location provisioned
> >>>>>>>>>> + by the guest in corresponding registers/table entries.
> >>>>>>>>>> +
> >>>>>>>>>> + The layout of the tables in guest memory defines an ABI. The entries
> >>>>>>>>>> + are laid out in little endian format as described in the last paragraph.
> >>>>>>>>>> +
> >>>>>>>>>> + KVM_DEV_ARM_ITS_RESTORE_TABLES
> >>>>>>>>>> + restore the ITS tables from guest RAM to ITS internal structures.
> >>>>>>>>>> +
> >>>>>>>>>> + The GICV3 must be restored before the ITS and all ITS registers but
> >>>>>>>>>> + the GITS_CTLR must be restored before restoring the ITS tables.
> >>>>>>>>>> +
> >>>>>>>>>> + The GITS_IIDR read-only register must also be restored before
> >>>>>>>>>> + the table restore as the IIDR revision field encodes the ABI revision.
> >>>>>>>>>> +
> >>>>>>>>>
> >>>>>>>>> what is the expected sequence of operations. For example, to restore
> >>>>>>>>> the ITS, do I call KVM_DEV_ARM_VGIC_CTRL_INIT first, then restore all
> >>>>>>>>> the memory and registers, and finally call KVM_DEV_ARM_ITS_RESTORE_TABLES?
> >>>>>>>> Yes KVM_DEV_ARM_VGIC_CTRL_INIT comes first, then restore all registers
> >>>>>>>> except GITS_CTLR, then table restore, then GITS_CTLR
> >>>>>>>>>
> >>>>>>>>> Is there any interaction between when you call KVM_DEV_ARM_ITS_RESTORE_TABLES
> >>>>>>>>> and restore GITS_CTLR (which enables the ITS)?
> >>>>>>>>
> >>>>>>>> Yep, when GITS_CTLR is set, LPIs may be enabled and this on that event
> >>>>>>>> that the pending table is read. But the whole pending table is not read
> >>>>>>>> as we only iterate on registered LPIs. So the ITT must have been
> >>>>>>>> restored previously.
> >>>>>>>>
> >>>>>>>> I became aware that the pending table sync is done twice, once in the
> >>>>>>>> pending table restore, and once in the GITS_CTLR restore. So if we
> >>>>>>>> leave this order specification, I should be able to remove the sync on
> >>>>>>>> table restore. This was the original reason why GITS_CTLR restore has
> >>>>>>>> been done at the very end.
> >>>>>>>
> >>>>>>> I'm sorry, I'm a bit confused. Do we not need
> >>>>>>> KVM_DEV_ARM_ITS_RESTORE_TABLES at all then?
> >>>>>>
> >>>>>> Yes you do. I was talking about the RDIST pending table sync. The save
> >>>>>> is explicit using GICV3 device KVM_DEV_ARM_VGIC_SAVE_PENDING_TABLES.
> >>>>>> However the sync is implicit on GITS_CTLR restore if LPIs are enabled.
> >>>>>> and today I do it also on ITS device KVM_DEV_ARM_ITS_RESTORE_TABLES
> >>>>>> which is not requested I think since GITS_CTLR restore does it already.
> >>>>>
> >>>>> Shouldn't restoring the pending tables happen when restoring some
> >>>>> redeistributor state and not anything related to the ITS?
> >>>>
> >>>> Marc wrote:
> >>>> "
> >>>> I don't think you necessarily need a coarse map. When restoring the ITS
> >>>> tables, you can always read the pending bit when creating the LPI
> >>>> structure (it has been written to RAM at save time). Note that we
> >>>> already do something like this in vgic_enable_lpis().
> >>>> "
> >>>>
> >>>> This is currently what is implemented I think. the pending tables are
> >>>> currently sync'ed on GITS_CTLR set (if LPI are enabled) + erroneously
> >>>> also on on ITS table restore
> >>>>
> >>>> The problematic is: Either you know in advance which LPI INTIDare used
> >>>> or you need to parse the whole pending table (possibly using the 1st kB
> >>>> as coarse mapping).
> >>>>
> >>>> If you don't know the LPI INTIDs in advance it is only possible to
> >>>> restore the pending bit of pending LPIs. At that time you would
> >>>> re-allocate those pending LPI (vgic_add_lpi) and when you restore the
> >>>> ITS ITT you would do the same for those which were not pending. Looks
> >>>> really heavy to me: coarse mapping + dual vgic_add_lpi path.
> >>>>
> >>>> Otherwise we would need to add another dependency between RDIST pending
> >>>> table restore and ITS table restore but this looks even more weird, no?
> >>>>
> >>>>
> >>> So I just sat down with Andre and Marc and we tried to work through this
> >>> and came up with the best scheme. I apologize in advance for the
> >>> one-way nature of this e-mail, and I am of course open to discussing the
> >>> following proposal again if you do not agree.
> >>>
> >>> What I think this document should say, is that the following ordering
> >>> must be followed when restoring the GIC and the ITS:
> >>>
> >>> First, restore all guest memory
> >>>
> >>> Second, restore ALL redistributors
> >>>
> >>> Third, restore the ITS, in the following order:
> >>> 1. Initialize the ITS (KVM_DEV_ARM_VGIC_CTRL_INIT)
> >>> 2. Restore GITS_CBASER
> >>> 3. Restore all other GITS_ registers, except GITS_CTLR!
> >>> 4. Load the ITS table data (KVM_DEV_ARM_ITS_RESTORE_TABLES)
> >>> 5. Restore GITS_CTLR
> >>>
> >>> The rationale is that we really want the redistributor and the ITS
> >>> restore to be independent and follow the architecture. This means that
> >>> our ABI for the redistributor should still work without restoring an ITS
> >>> (if we ever decide to support LPIs for KVM without the ITS).
> >>
> >> OK. Note I already mentioned that GICv3 must be restored before the ITS.
> >> To me this comprised the RDIST.
> >>
> >> I understand the above description of the ordering comes in addition to
> >> the existing text, right? in other words I keep the GITS_READR,
> >> GITS_IIDR specific text as well as KVM_DEV_ARM_ITS_SAVE/RESTORE_TABLES
> >> section.
> >>
> >>>
> >>> In terms of our current implementation this means that vgic_add_lpi()
> >>> should ask the redistributor what the state of the LPI is (priority,
> >>> enabled, pending).
> >> this practically means I move update_lpi_config call from
> >> vgic_its_restore_ite to vgic_add_lpi(). OK
> >>
> >> However for getting the LPI pending state I must know which RDIST the
> >> LPI is attached to. This is not known at LPI allocation time. Do I
> >> misunderstand something?
> >
> > Once you have rebuilt the ITS data structures and allocated the IRQ
> > structures, you should have a target_cpu field pointing to the right
> > vcpu. From there, you can surely find the corresponding redistributor
> > and the pending table.
> Yes that's understood but Christoffer said "vgic_add_lpi() should ask
> the redistributor what the state of the LPI is (priority,enabled,
> pending)." Fetching the properties is fine.
>
> vgic_add_lpi() is called before update_affinity_ite() which uses
> ite->irq and sets the target_vcpu.
>
> Well at least this requires some function reshape. I will investigate
> though.

Thanks. If it looks impossible, let me know, and I can help having a
look at the code.

-Christoffer