linux-kernel - Re: [PATCH] KVM: arm/arm64: vgic-its: Check hopefully the last DISCARD command error

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Date:   Tue, 14 Jan 2020 09:20:50 +0100
From:   Auger Eric <eric.auger@...hat.com>
To:     Zenghui Yu <yuzenghui@...wei.com>, maz@...nel.org
Cc:     andre.przywara@....com, wanghaibin.wang@...wei.com,
        kvmarm@...ts.cs.columbia.edu, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: arm/arm64: vgic-its: Check hopefully the last
 DISCARD command error

Hi Zenghui,

On 1/14/20 8:10 AM, Zenghui Yu wrote:
> Hi Eric,
> 
> On 2020/1/10 16:37, Auger Eric wrote:
>> Hi Zenghui,
>>
>> On 12/25/19 2:30 PM, Zenghui Yu wrote:
>>> DISCARD command error occurs if any of the following apply:
>>>
>>>   - [ ... (those which we have already handled) ]
>> nit: I would remove the above and simply say the discard is supposed to
>> fail if the collection is not mapped to any target redistributor. If an
>> ITE exists then the ite->collection is non NULL.
> 
> I think this is not always true. Let's talk about the following scenario
> (a bit insane, though):
> 
> 1. First map a LPI to an unmapped Collection, then ite->collection is
>    non NULL and its target_addr is COLLECTION_NOT_MAPPED.
> 
> 2. Then issue MAPC and unMAPC(V=0) commands on this Collection, the
>    ite->collection will be NULL, see vgic_its_free_collection().
You're right I missed that case.
> 
> Discard the LPI mapping after "1" or "2", we will both encounter the
> unmapped collection command error.
> 
>> What needs to be checked is its_is_collection_mapped().
>>
>> By the way update_affinity_collection() also tests ite->collection. I
>> think this is useless or do I miss something?
> 
> Yeah, I agree. We managed to invoke update_affinity_collection(,, coll),
> ensure that the 'coll' can _not_ be NULL.
> So '!ite->collection' is already a subcase of 'coll != ite->collection'.
> We can safely get rid of it.
OK. But that's not for the (wrong) reason I mentioned above. So it is a
minor cleanup and you may just leave it as is and just focus on this fix.

Thanks

Eric
> 
>>
>> Reviewed-by: Eric Auger <eric.auger@...hat.com>
>>
> 
> Thanks for that. I'll change the commit message with your suggestion and
> add your R-b in v2.
> 
> 
> Thanks,
> Zenghui
> 
> 
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