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

Auger Eric eric.auger at redhat.com
Thu Apr 27 08:51:00 EDT 2017

Hi Christoffer,

On 27/04/2017 13:02, Christoffer Dall wrote:
> On Thu, Apr 27, 2017 at 11:33:39AM +0200, Auger Eric wrote:
>> Hi Christoffer,
>> On 27/04/2017 10:57, Christoffer Dall wrote:
>>> On Wed, Apr 26, 2017 at 05:48:32PM +0200, Auger Eric wrote:
>>>> Hi Christoffer,
>>>> 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 at 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:
>>>>>>        request the initialization of the ITS, no additional parameter in
>>>>>>        kvm_device_attr.addr.
>>>>>> +
>>>>>> +      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.
>>>>>> +
>>>>>> +      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
>> 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?

>> KVM_DEV_ARM_ITS_RESTORE_TABLES restores all the ITS tables (device,
>> collection, ITT)
> Why do you need this if you anyway need to restore the CTLR as the last
> thing?  Just to make it absolutely clear when it happens, or is there
> something which has to happen between the CTLR and the RESTORE?

Not sure I get this one.

by doing the lookup of Device table and ITT,
KVM_DEV_ARM_ITS_RESTORE_TABLES enumerates all the LPI INTIDs that exist,
re-allocate them

Then when GITS_CTLR is set, you can use those INTIDs to parse the exact
pending status in the pending tables. In case
KVM_DEV_ARM_ITS_RESTORE_TABLES were not called you would be "blind" and
you would need to parse the whole pending table.


> Thanks,
> -Christoffer

More information about the linux-arm-kernel mailing list