Re: [PATCH v2 10/27] ARM: GICv3: forward pending LPIs to guests

From: Julien Grall <julien.grall@arm.com>
To: Andre Przywara <andre.przywara@arm.com>,
	Stefano Stabellini <sstabellini@kernel.org>
Cc: xen-devel@lists.xenproject.org,
	Shanker Donthineni <shankerd@codeaurora.org>,
	Vijay Kilari <vijay.kilari@gmail.com>
Subject: Re: [PATCH v2 10/27] ARM: GICv3: forward pending LPIs to guests
Date: Tue, 4 Apr 2017 13:50:18 +0100	[thread overview]
Message-ID: <8d980515-beeb-8581-a3b3-6ff7668610b2@arm.com> (raw)
In-Reply-To: <e22e705e-50a8-ff15-1c14-9db75e99f5db@arm.com>

On 04/04/17 13:51, Andre Przywara wrote:
> On 04/04/17 12:49, Julien Grall wrote:
>>
>>
>> On 03/04/17 15:18, Andre Przywara wrote:
>>> Hi,
>>
>> Hi Andre,
>>
>>> On 24/03/17 12:03, Julien Grall wrote:
>>>>> +    /* We may have mapped more host LPIs than the guest actually
>>>>> asked for. */
>>>>
>>>> Another way, is the interrupt has been received at the same time the
>>>> guest is configuring it. What will happen if the interrupt is lost?
>>>
>>> I don't see how this would be our problem. If the guest is still about
>>> to configure an LPI, then any incoming one is pretty clearly a spurious
>>> interrupt.
>>> I take it that you mean "mapping an LPI using ITS commands" when you say
>>> "configuring" here. If this is not what you mean, please correct me.
>>>
>>> I'd expect a guest to first map the LPI, then enable it and send an INV
>>> command. Doing that differently will not result in any sane behavior (as
>>> in: a guest cannot expect an LPI to come through), so nothing we need to
>>> worry about (from a lost IRQ perspective).
>>>
>>> Or what do I miss here?
>>
>> All host LPIs will be enabled when a device is assigned to a guest. So
>> technically an LPI can come at anytime before the guest is configuring
>> the virtual mapping.
>
> As I said: what is the problem? Any LPI coming in without being mapped
> by a guest would be spurious from a guest point of view, as in not
> expected and ignored. But a guest will never receive it, since we
> explicitly check for that and on the host side and bail out early. From
> do_LPI():
>     /* Unmapped events are marked with an invalid LPI ID. */
>     if ( hlpi.virt_lpi == INVALID_LPI )
>         return;
>
> And we (atomically) update the entry once the guest has configured it,
> so either it's ignored or delivered. And this is a benign race you have
> on real hardware too.

The document it... if I ask a questions multiple time it is likely the 
code is not clear enough.

Cheers,

-- 
Julien Grall

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