Re: [RFC 02/12] KVM: arm/arm64: Document KVM_VGIC_V3_ADDR_TYPE_REDIST_REGION

From: Marc Zyngier
Date: Mon Mar 19 2018 - 10:15:59 EST


On 19/03/18 09:20, Eric Auger wrote:
> We introduce a new KVM_VGIC_V3_ADDR_TYPE_REDIST_REGION attribute in
> KVM_DEV_ARM_VGIC_GRP_ADDR group. It allows userspace to provide the
> base address and size of a redistributor region
>
> Compared to KVM_VGIC_V3_ADDR_TYPE_REDIST, this new attribute allows
> to declare several separate redistributor regions.
>
> So the whole redist space does not need to be contiguous anymore.
>
> Signed-off-by: Eric Auger <eric.auger@xxxxxxxxxx>
> ---
> Documentation/virtual/kvm/devices/arm-vgic-v3.txt | 12 ++++++++++++
> 1 file changed, 12 insertions(+)
>
> diff --git a/Documentation/virtual/kvm/devices/arm-vgic-v3.txt b/Documentation/virtual/kvm/devices/arm-vgic-v3.txt
> index 9293b45..2c0bedf 100644
> --- a/Documentation/virtual/kvm/devices/arm-vgic-v3.txt
> +++ b/Documentation/virtual/kvm/devices/arm-vgic-v3.txt
> @@ -27,6 +27,18 @@ Groups:
> VCPU and all of the redistributor pages are contiguous.
> Only valid for KVM_DEV_TYPE_ARM_VGIC_V3.
> This address needs to be 64K aligned.
> +
> + KVM_VGIC_V3_ADDR_TYPE_REDIST_REGION (rw, 64-bit)
> + The attr field of kvm_device_attr encodes 3 values:
> + bits: | 63 .... 52 | 51 .... 12 |11 - 0
> + values: | pfns | base | index
> + - index encodes the unique redistibutor region index
> + - base field encodes bits [51:12] the guest physical base address
> + of the first redistributor in the region. There are two 64K pages
> + for each VCPU and all of the redistributor pages are contiguous
> + within the redistributor region.

Why does base have to include bits [15:12] of the IPA? If it is 64kB
aligned (as it should), these bits are guaranteed to be 0. This also
avoid having to return -EINVAL in case of alignment problems.

> + - pfns encodes the size of the region in 64kB pages.
> + Only valid for KVM_DEV_TYPE_ARM_VGIC_V3.

"pfns" a bit of an odd name, and is a bit unnatural. Why don't we
instead encode the number of redistributors? You've already imposed that
a redistributor has 2 64kB pages, so let's take advantage of this by
saving yet another bit.

You could dedicate a couple of these bits as a selector for the ITS
version (v3, v4 or whatever comes after that, if ever...). Just make 0
the only valid version for the time being.

> Errors:
> -E2BIG: Address outside of addressable IPA range
> -EINVAL: Incorrectly aligned address
>

Thanks,

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