linux-kernel - Re: [PATCH v2 2/5] iommu/virtio: Support bypass domains

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Date:   Mon, 29 Nov 2021 15:36:47 +0000
From:   Jean-Philippe Brucker <jean-philippe@...aro.org>
To:     Eric Auger <eric.auger@...hat.com>
Cc:     joro@...tes.org, will@...nel.org, mst@...hat.com,
        jasowang@...hat.com, virtualization@...ts.linux-foundation.org,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        sebastien.boeuf@...el.com, kevin.tian@...el.com,
        pasic@...ux.ibm.com
Subject: Re: [PATCH v2 2/5] iommu/virtio: Support bypass domains

On Sat, Nov 27, 2021 at 05:18:28PM +0100, Eric Auger wrote:
> Hi Jean,
> 
> On 11/23/21 4:52 PM, Jean-Philippe Brucker wrote:
> > The VIRTIO_IOMMU_F_BYPASS_CONFIG feature adds a new flag to the ATTACH
> > request, that creates a bypass domain. Use it to enable identity
> > domains.
> >
> > When VIRTIO_IOMMU_F_BYPASS_CONFIG is not supported by the device, we
> > currently fail attaching to an identity domain. Future patches will
> > instead create identity mappings in this case.
> >
> > Reviewed-by: Kevin Tian <kevin.tian@...el.com>
> > Signed-off-by: Jean-Philippe Brucker <jean-philippe@...aro.org>
> > ---
> >  drivers/iommu/virtio-iommu.c | 20 +++++++++++++++++++-
> >  1 file changed, 19 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/iommu/virtio-iommu.c b/drivers/iommu/virtio-iommu.c
> > index 80930ce04a16..ee8a7afd667b 100644
> > --- a/drivers/iommu/virtio-iommu.c
> > +++ b/drivers/iommu/virtio-iommu.c
> > @@ -71,6 +71,7 @@ struct viommu_domain {
> >  	struct rb_root_cached		mappings;
> >  
> >  	unsigned long			nr_endpoints;
> > +	bool				bypass;
> >  };
> >  
> >  struct viommu_endpoint {
> > @@ -587,7 +588,9 @@ static struct iommu_domain *viommu_domain_alloc(unsigned type)
> >  {
> >  	struct viommu_domain *vdomain;
> >  
> > -	if (type != IOMMU_DOMAIN_UNMANAGED && type != IOMMU_DOMAIN_DMA)
> > +	if (type != IOMMU_DOMAIN_UNMANAGED &&
> > +	    type != IOMMU_DOMAIN_DMA &&
> > +	    type != IOMMU_DOMAIN_IDENTITY)
> >  		return NULL;
> >  
> >  	vdomain = kzalloc(sizeof(*vdomain), GFP_KERNEL);
> > @@ -630,6 +633,17 @@ static int viommu_domain_finalise(struct viommu_endpoint *vdev,
> >  	vdomain->map_flags	= viommu->map_flags;
> >  	vdomain->viommu		= viommu;
> >  
> > +	if (domain->type == IOMMU_DOMAIN_IDENTITY) {
> > +		if (!virtio_has_feature(viommu->vdev,
> nit: couldn't the check be done before the ida_alloc_range(),
> simplifying the failure cleanup?

It could, but patch 5 falls back to identity mappings, which is better
left at the end of the function to keep the error path simple. I put this
at the end already here, so patch 5 doesn't need to move things around.

Thanks,
Jean