linux-kernel - Re: [RFC] KVM: arm/arm64: Introduce KVM_DEV_ARM_ITS_CTRL

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Date:   Fri, 15 Sep 2017 14:26:41 +0200
From:   Auger Eric <eric.auger@...hat.com>
To:     Marc Zyngier <marc.zyngier@....com>
Cc:     eric.auger.pro@...il.com, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, cdall@...aro.org, peter.maydell@...aro.org,
        andre.przywara@....com, wanghaibin.wang@...wei.com,
        wu.wubin@...wei.com
Subject: Re: [RFC] KVM: arm/arm64: Introduce KVM_DEV_ARM_ITS_CTRL_RESET

Hi,

On 14/09/2017 19:06, Marc Zyngier wrote:
> On Thu, Sep 14 2017 at 10:57:28 am BST, Eric Auger <eric.auger@...hat.com> wrote:
>> At the moment, the in-kernel emulated ITS is not properly reset.
>> On guest restart/reset some registers keep their old values and
>> internal structures like device, ITE, collection lists are not emptied.
>>
>> This may lead to various bugs. Among them, we can have incorrect state
>> backup or failure when saving the ITS state at early guest boot stage.
>>
>> This patch introduces a new attribute, KVM_DEV_ARM_ITS_CTRL_RESET in
>> the KVM_DEV_ARM_VGIC_GRP_CTRL group.
>>
>> Upon this action, we can invalidate the various memory structures
>> pointed by GITS_BASERn and GITS_CBASER, free the ITS internal caches
>> and reset the relevant registers.
>>
>> Signed-off-by: Eric Auger <eric.auger@...hat.com>
>>
>> ---
>>
>> An alternative would consist in having the userspace writing
>> individual registers with default values: GITS_BASERn, GITS_CBASER
>> and GITS_CTLR. On kernel side we would reset related lists when
>> detecting the valid bit is set to false.
> 
> I'm not sure this is necessarily a "either/or" situation. It looks to me
> that we're not completely doing the right thing when writing to the
> GITS_BASER registers, and that writing a new value (with the valid bit
> set or not) should have an action of some sort on the fate of the
> existing mappings.

I agree. I think whenever the GITS_BASERn or GITS_CBASER validity bit is
reset, we should empty the internal lists and assure the code does not
attempt to read the data structures in caches/RAM anymore.

I will follow up with some patches.

Thanks

Eric
> 
> Thoughts?
> 
> 	M.
>