linux-kernel - RE: [PATCH v9 04/12] vfio iommu: Add support for mediated devices

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Date:   Wed, 26 Oct 2016 07:53:43 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Alex Williamson <alex.williamson@...hat.com>,
        Kirti Wankhede <kwankhede@...dia.com>
CC:     "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "kraxel@...hat.com" <kraxel@...hat.com>,
        "cjia@...dia.com" <cjia@...dia.com>,
        "qemu-devel@...gnu.org" <qemu-devel@...gnu.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "Song, Jike" <jike.song@...el.com>,
        "bjsdjshi@...ux.vnet.ibm.com" <bjsdjshi@...ux.vnet.ibm.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v9 04/12] vfio iommu: Add support for mediated devices

> From: Alex Williamson [mailto:alex.williamson@...hat.com]
> Sent: Thursday, October 20, 2016 5:03 AM
> > @@ -83,6 +92,21 @@ struct vfio_group {
> >  };
> >
> >  /*
> > + * Guest RAM pinning working set or DMA target
> > + */
> > +struct vfio_pfn {
> > +	struct rb_node		node;
> > +	unsigned long		vaddr;		/* virtual addr */
> > +	dma_addr_t		iova;		/* IOVA */
> > +	unsigned long		pfn;		/* Host pfn */
> > +	int			prot;
> > +	atomic_t		ref_count;
> > +};
> 
> Somehow we're going to need to fit an invalidation callback here too.
> How would we handle a case where there are multiple mdev devices, from
> different vendor drivers, that all have the same pfn pinned?  I'm
> already concerned about the per pfn overhead we're introducing here so
> clearly we cannot store an invalidation callback per pinned page, per
> vendor driver.  Perhaps invalidations should be done using a notifier
> chain per vfio_iommu, the vendor drivers are required to register on
> that chain (fail pinning with empty notifier list) user unmapping
> will be broadcast to the notifier chain, the vendor driver will be
> responsible for deciding if each unmap is relevant to them (potentially
> it's for a pinning from another driver).
> 
> I expect we also need to enforce that vendors perform a synchronous
> unmap such that after returning from the notifier list call, the
> vfio_pfn should no longer exist.  If it does we might need to BUG_ON.
> Also be careful to pay attention to the locking of the notifier vs
> unpin callbacks to avoid deadlocks.
> 

What about just requesting vendor driver to provide a callback in parent 
device ops? 

Curious in which scenario the user application (say Qemu here) may 
unmap memory pages which are still pinned by vendor driver... Is it 
purely about a corner case which we want to handle elegantly? 

If yes, possibly a simpler way is to force destroying mdev instead of 
asking vendor driver to take care of each invalidation request under
such situation. Since anyway the mdev device won't be in an usable
state anymore... (sorry if I missed the key problem here.)

Thanks
Kevin