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Re: [Xen-devel] Xen/arm: Virtual ITS command queue handling

On 19/05/15 13:48, Vijay Kilari wrote:
> On Tue, May 19, 2015 at 5:49 PM, Ian Campbell <ian.campbell@xxxxxxxxxx> wrote:
>> On Tue, 2015-05-19 at 17:40 +0530, Vijay Kilari wrote:
>>>> If a guest issues (for example) a MOVI which is not followed by an
>>>> INV/INVALL on native then what would trigger the LPI configuration to be
>>>> applied by the h/w?
>>>> If a guest is required to send an INV/INVALL in order for some change to
>>>> take affect and it does not do so then it is buggy, isn't it?
>>> agreed.
>>>> IOW all Xen needs to do is to propagate any guest initiated INV/INVALL
>>>> as/when it occurs in the command queue. I don't think we need to
>>>> fabricate an additional INV/INVALL while emulating a MOVI.
>>>> What am I missing?
>>> back to point:
>>> INV has device id so not an issue.
>>> INVALL does not have device id to know pITS to send.
>>> For that reason Xen is expected to insert INVALL at proper
>>> places similar to SYNC and ignore INV/INVALL of guest.
>> Why wouldn't Xen just insert an INVALL in to all relevant pITS in
>> response to an INVALL from the guest?
> If INVALL is sent on all pITS, then we need to wait for all pITS to complete
> the command before we update CREADR of vITS.
>> If you are proposing something different then please be explicit by what
>> you mean by "proper places similar to SYNC". Ideally by proposing some
>> new text which I can use in the document.
> If the platform has more than 1 pITS, The ITS commands are mapped
> from vITS to pITS using device ID provided with ITS command.
> However SYNC and INVALL does not have device ID.
> In such case there could be two ways to handle
> 1) SYNC and INVALL of guest will be sent to pITS based on previous ITS 
> commands
>     of guest
> 2) Xen will insert/append SYNC and INVALL to guest ITS commands
> where-ever required and ignore guest
>    SYNC and INVALL commands
> IMO (2) would be better as approach (1) might fail to handle
> scenario where-in guest is sending only SYNC & INVALL commands.

When the guest send a SYNC, it expects all the command to be completed.
If you send SYNC only when you think it's required we will end up to
unexpected behavior.

Now, for INVALL, as said on a previous mail it's never required after an
instruction. It's used to ask the ITS to invalid his cache of the LPI

A software would be buggy if no INV/INVALL is sent after change the LPI
configuration table.

As suggested on a previous mail, I think we can get rid of sending
INV/INVALL command to the pITS by trapping the LPI configuration table:

For every write access, when the vLPIs is valid (i.e associated to a
device/interrupt), Xen will toggle the enable bit in the hardware LPIs
configuration table, send an INV * and sync his internal state. This
requiring to be able to translate the vLPIs to a (device,ID).

INVALL/INV command could be ignored and directly increment CREADR (with
some care) because it only ensure that the command has been executed,
not fully completed. A SYNC would be required from the guest in order to
ensure the completion.

Therefore we would need more care for the SYNC. Maybe by injecting a
SYNC when it's necessary.

Note that we would need Xen to send command on behalf of the guest (i.e
not part of the command queue).

With this solution, it would be possible to have a small amount of time
where the pITS doesn't use the correct the configuration (i.e the
interrupt not yet enabled/disabled). Xen is able to cooperate with that
and will queue the interrupt to the guest.


Julien Grall

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