linux-kernel - Re: [PATCH v2 1/2] iommu/virtio: Make use of ops->iotlb_sync

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Message-ID: <20230922075230.GA1361815@myrica>
Date:   Fri, 22 Sep 2023 08:52:30 +0100
From:   Jean-Philippe Brucker <jean-philippe@...aro.org>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Niklas Schnelle <schnelle@...ux.ibm.com>,
        Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
        virtualization@...ts.linux-foundation.org, iommu@...ts.linux.dev,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] iommu/virtio: Make use of ops->iotlb_sync_map

On Tue, Sep 19, 2023 at 09:28:08AM +0100, Robin Murphy wrote:
> On 2023-09-19 09:15, Jean-Philippe Brucker wrote:
> > On Mon, Sep 18, 2023 at 05:37:47PM +0100, Robin Murphy wrote:
> > > > diff --git a/drivers/iommu/virtio-iommu.c b/drivers/iommu/virtio-iommu.c
> > > > index 17dcd826f5c2..3649586f0e5c 100644
> > > > --- a/drivers/iommu/virtio-iommu.c
> > > > +++ b/drivers/iommu/virtio-iommu.c
> > > > @@ -189,6 +189,12 @@ static int viommu_sync_req(struct viommu_dev *viommu)
> > > >    	int ret;
> > > >    	unsigned long flags;
> > > > +	/*
> > > > +	 * .iotlb_sync_map and .flush_iotlb_all may be called before the viommu
> > > > +	 * is initialized e.g. via iommu_create_device_direct_mappings()
> > > > +	 */
> > > > +	if (!viommu)
> > > > +		return 0;
> > > 
> > > Minor nit: I'd be inclined to make that check explicitly in the places where
> > > it definitely is expected, rather than allowing *any* sync to silently do
> > > nothing if called incorrectly. Plus then they could use
> > > vdomain->nr_endpoints for consistency with the equivalent checks elsewhere
> > > (it did take me a moment to figure out how we could get to .iotlb_sync_map
> > > with a NULL viommu without viommu_map_pages() blowing up first...)
> > 
> > They're not strictly equivalent: this check works around a temporary issue
> > with the IOMMU core, which calls map/unmap before the domain is finalized.
> > Once we merge domain_alloc() and finalize(), then this check disappears,
> > but we still need to test nr_endpoints in map/unmap to handle detached
> > domains (and we still need to fix the synchronization of nr_endpoints
> > against attach/detach). That's why I preferred doing this on viommu and
> > keeping it in one place.
> 
> Fair enough - it just seems to me that in both cases it's a detached domain,
> so its previous history of whether it's ever been otherwise or not shouldn't
> matter. Even once viommu is initialised, does it really make sense to send
> sync commands for a mapping on a detached domain where we haven't actually
> sent any map/unmap commands?

If no requests were added by map/unmap, then viommu_sync_req() is
essentially a nop because virtio doesn't use sync commands (and
virtqueue_kick() only kicks the host when the queue's not empty, if I
remember correctly). It still does a bit of work so is less efficient than
a preliminary check on nr_endpoints, but it feels nicer to streamline the
case where the domain is attached, since map/unmap on detached domains
happens rarely, if ever.

Either is fine by me. An extra test won't make much difference performance
wise, and I guess will look less confusing. Niklas, do you mind resending
the version with nr_endpoints check (and updated commit messages)?

Thanks,
Jean