linux-kernel - Re: [RFC 0/4] Virtio uses DMA API for all devices

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Message-ID: <20180803160246.GA13794@infradead.org>
Date:   Fri, 3 Aug 2018 09:02:46 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:     Christoph Hellwig <hch@...radead.org>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Will Deacon <will.deacon@....com>,
        Anshuman Khandual <khandual@...ux.vnet.ibm.com>,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        aik@...abs.ru, robh@...nel.org, joe@...ches.com,
        elfring@...rs.sourceforge.net, david@...son.dropbear.id.au,
        jasowang@...hat.com, mpe@...erman.id.au, linuxram@...ibm.com,
        haren@...ux.vnet.ibm.com, paulus@...ba.org,
        srikar@...ux.vnet.ibm.com, robin.murphy@....com,
        jean-philippe.brucker@....com, marc.zyngier@....com
Subject: Re: [RFC 0/4] Virtio uses DMA API for all devices

On Fri, Aug 03, 2018 at 10:58:36AM -0500, Benjamin Herrenschmidt wrote:
> On Fri, 2018-08-03 at 00:05 -0700, Christoph Hellwig wrote:
> > >   2- Make virtio use the DMA API with our custom platform-provided
> > > swiotlb callbacks when needed, that is when not using IOMMU *and*
> > > running on a secure VM in our case.
> > 
> > And total NAK the customer platform-provided part of this.  We need
> > a flag passed in from the hypervisor that the device needs all bus
> > specific dma api treatment, and then just use the normal plaform
> > dma mapping setup. 
> 
> Christoph, as I have explained already, we do NOT have a way to provide
> such a flag as neither the hypervisor nor qemu knows anything about
> this when the VM is created.

Well, if your setup is so fucked up I see no way to support it in Linux.

Let's end the discussion right now then.