Re: [RFC] KVM: arm/arm64: Introduce KVM_DEV_ARM_ITS_CTRL_RESET

[Christoffer Dall]

On Thu, Sep 14, 2017 at 10:57:28AM +0200, Eric Auger 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

It's more about freeing the cached data structures than what the BASERn
registers point to, really, but ok.

> and reset the relevant registers.
> 
> Signed-off-by: Eric Auger <eric.auger@xxxxxxxxxx>
> 
> ---
> 
> 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 crazy about that idea.

> ---
>  Documentation/virtual/kvm/devices/arm-vgic-its.txt | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/Documentation/virtual/kvm/devices/arm-vgic-its.txt b/Documentation/virtual/kvm/devices/arm-vgic-its.txt
> index eb06beb..ebb15c5 100644
> --- a/Documentation/virtual/kvm/devices/arm-vgic-its.txt
> +++ b/Documentation/virtual/kvm/devices/arm-vgic-its.txt
> @@ -33,6 +33,9 @@ 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.
> +

I can't find information in the spec about what 'reset the ITS' means.
So I think we need to describe this a little more carefully.  Which
assumptions does a user have after calling this.

>      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.
> -- 
> 2.5.5
> 

Thanks,
-Christoffer