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

[Peter Maydell]

On 19 March 2018 at 09:20, Eric Auger <eric.auger@xxxxxxxxxx> 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

"redistributor"

> +      - base field encodes bits [51:12] the guest physical base address

"of the guest"

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

You should say something here about what happens if userspace
tries to use both KVM_VGIC_V3_ADDR_TYPE_REDIST_REGION and
KVM_VGIC_V3_ADDR_TYPE_REDIST. I think this should be an error
(reported by whichever of the two you try to set second).

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

Marc wrote:
> 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.

If you're not using the bits for anything else you want to
check they're 0 anyway. Otherwise we can't guarantee to safely
use them for something else in future, because userspace might
be handing us garbage in those bits without noticing.

thanks
-- PMM