Re: [PATCH] intel_iommu: support snoop control

[Peter Xu]

On Mon, Feb 14, 2022 at 02:03:46PM +0800, Jason Wang wrote:
> SC is required for some kernel features like vhost-vDPA. So this patch
> implements basic SC feature. The idea is pretty simple, for software
> emulated DMA it would be always coherent. In this case we can simple
> advertise ECAP_SC bit. For VFIO and vhost, thing will be more much
> complicated, so this patch simply fail the IOMMU notifier
> registration.

Could we spell out which vhost branch won't work?  How about also mention what
this patch is used for (perhaps for some pure vdpa tests on fully emulated)?

> 
> In the future, we may want to have a dedicated notifiers flag or
> similar mechanism to demonstrate the coherency so VFIO could advertise
> that if it has VFIO_DMA_CC_IOMMU, for vhost kernel backend we don't
> need that since it's a software backend.
> 
> Signed-off-by: Jason Wang <jasowang@redhat.com>
> ---
>  hw/i386/intel_iommu.c          | 14 +++++++++++++-
>  hw/i386/intel_iommu_internal.h |  1 +
>  include/hw/i386/intel_iommu.h  |  1 +
>  3 files changed, 15 insertions(+), 1 deletion(-)
> 
> diff --git a/hw/i386/intel_iommu.c b/hw/i386/intel_iommu.c
> index 5b865ac08c..5fa8e361b8 100644
> --- a/hw/i386/intel_iommu.c
> +++ b/hw/i386/intel_iommu.c
> @@ -3022,6 +3022,13 @@ static int 
> vtd_iommu_notify_flag_changed(IOMMUMemoryRegion *iommu,
>      VTDAddressSpace *vtd_as = container_of(iommu, VTDAddressSpace, iommu);
>      IntelIOMMUState *s = vtd_as->iommu_state;
>  
> +    /* TODO: add support for VFIO and vhost users */
> +    if (s->snoop_control) {
> +        error_setg_errno(errp, -ENOTSUP,
> +                         "Snoop Control with vhost or VFIO is not 
> supported");
> +        return -ENOTSUP;
> +    }

IIUC this will also fail things like e.g. vhost-kernel but afaiu that can be
fully emulated too.  That's expected, am I right?

Thanks,

> +
>      /* Update per-address-space notifier flags */
>      vtd_as->notifier_flags = new;

-- 
Peter Xu