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

[eric.auger@redhat.com (Auger Eric)]

Hi Christoffer,

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).
> 
> 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).  I suggest you do the pending check by adding a
> function called something like vgic_v3_lpi_is_pending() which scans the
> bit in memory, clears the memory bit, and returns the value.  Clearing
> the pending bit in memory when moving it to the struct irq is nice,
> because you then don't have to clear out the entire pending table later
> and we don't keep 'consumed' data lying around.  This change should be
> implemented in its_sync_lpi_pending_table() as well, but note that you
> need never call that function in the normal restore path using this
> design.
> 
> I hope this makes sense.

I am dubious about the above changes at the moment.
its_sync_lpi_pending_table() gets called on GITS_CTLR setting which is
documented to be the last step of the restoration. I wonder why the
above changes cannot be part of another series later on.

Consuming the RAM bit status means we record it in irq->pending_latch so
I guess we should have the irq->pending_latch setting in the same
function as the one that retrieves the bit status in guest RAM. So I
would rename vgic_v3_lpi_is_pending into something like
int vgic_v3_sync_lpi_pending_status(struct kvm *kvm, u32 intid)
Since this covers a single LPI, the removes the byte access optimization
found in its_sync_lpi_pending_table

Also if I understand it correctly this means the sync will be done on
both add_lpi and GITS_CTLR setting

What do you think?

Thanks

Eric
> 
> Thanks,
> -Christoffer
> 
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