linux-kernel - Re: [PATCH v5 09/10] KVM: arm/arm64: Document KVM_DEV_ARM_ITS_CTRL

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Date:   Wed, 25 Oct 2017 11:40:22 +0100
From:   Marc Zyngier <marc.zyngier@....com>
To:     Eric Auger <eric.auger@...hat.com>
Cc:     <eric.auger.pro@...il.com>, <linux-kernel@...r.kernel.org>,
        <kvm@...r.kernel.org>, <kvmarm@...ts.cs.columbia.edu>,
        <cdall@...aro.org>, <peter.maydell@...aro.org>,
        <andre.przywara@....com>, <wanghaibin.wang@...wei.com>,
        <wu.wubin@...wei.com>, <drjones@...hat.com>, <wei@...hat.com>
Subject: Re: [PATCH v5 09/10] KVM: arm/arm64: Document KVM_DEV_ARM_ITS_CTRL_RESET

On Mon, Oct 23 2017 at  4:08:28 pm 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, and collection lists are not
> freed.
>
> 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 documents a new attribute, KVM_DEV_ARM_ITS_CTRL_RESET in
> the KVM_DEV_ARM_VGIC_GRP_CTRL group.
>
> Upon this action, we can reset registers and especially those
> pointing to tables previously allocated by the guest and free
> the internal data structures storing the list of devices, collections
> and lpis.
>
> The usual approach for device reset of having userspace write
> the reset values of the registers to the kernel via the register
> read/write APIs doesn't work for the ITS because it has some
> internal state (caches) which is not exposed as registers,
> and there is no register interface for "drop cached data without
> writing it back to RAM". So we need a KVM API which mimics the
> hardware's reset line, to provide the equivalent behaviour to
> a "pull the power cord out of the back of the machine" reset.
>
> Signed-off-by: Eric Auger <eric.auger@...hat.com>
> Reported-by: wanghaibin <wanghaibin.wang@...wei.com>
>
> ---
> v4 -> v5:
> - some rewording according to Christoffer's comments
>
> v2 -> v3:
> - reword commit message, credit to Peter Maydell.
> - take into account Christoffer rewording comments but still
>   kept details. Added Peter's comment but still kept details.
>   Peter may disagree.
>
> v1 -> v2:
> - Describe architecturally-defined reset values
> ---
>  Documentation/virtual/kvm/devices/arm-vgic-its.txt | 21 +++++++++++++++++++++
>  1 file changed, 21 insertions(+)
>
> diff --git a/Documentation/virtual/kvm/devices/arm-vgic-its.txt b/Documentation/virtual/kvm/devices/arm-vgic-its.txt
> index eb06beb..d12d8e9 100644
> --- a/Documentation/virtual/kvm/devices/arm-vgic-its.txt
> +++ b/Documentation/virtual/kvm/devices/arm-vgic-its.txt
> @@ -33,6 +33,10 @@ Groups:
>        request the initialization of the ITS, no additional parameter in
>        kvm_device_attr.addr.
>  
> +    KVM_DEV_ARM_ITS_CTRL_RESET
> +      reset the ITS, no additional parameter in kvm_device_attr.addr.
> +      See "ITS Reset State" section.
> +
>      KVM_DEV_ARM_ITS_SAVE_TABLES
>        save the ITS table data into guest RAM, at the location provisioned
>        by the guest in corresponding registers/table entries.
> @@ -157,3 +161,20 @@ Then vcpus can be started.
>   - pINTID is the physical LPI ID; if zero, it means the entry is not valid
>     and other fields are not meaningful.
>   - ICID is the collection ID
> +
> + ITS Reset State:
> + ----------------
> +
> +RESET returns the ITS to the same state that it was when first created and
> +initialized. When the RESET command returns, the following things are
> +guaranteed:
> +
> +- The ITS is not enabled and quiescent
> +  GITS_CTLR.Enabled = 0 .Quiescent=1
> +- There is no internally cached state
> +- No collection or device table are used
> +  GITS_BASER<n>.Valid = 0
> +- The command queue is not allocated:

I don't think we should say anything like that. Allocation is a guest
thing, and hasn't much to do with the ITS itself. Specifying the state
of the various registers should be enough.

> +  GITS_CBASER = 0, GITS_CREADR = 0, GITS_CWRITER = 0
> +- The ABI version is unchanged and remains the one set when the ITS
> +  device was first created.

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny.