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

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Date:   Fri, 03 Aug 2018 10:58:36 -0500
From:   Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     "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, 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.

>  To get swiotlb you'll need to then use the DT/ACPI
> dma-range property to limit the addressable range, and a swiotlb
> capable plaform will use swiotlb automatically.

This cannot be done as you describe it.

The VM is created as a *normal* VM. The DT stuff is generated by qemu
at a point where it has *no idea* that the VM will later become secure
and thus will have to restrict which pages can be used for "DMA".

The VM will *at runtime* turn itself into a secure VM via interactions
with the security HW and the Ultravisor layer (which sits below the
HV). This happens way after the DT has been created and consumed, the
qemu devices instanciated etc...

Only the guest kernel knows because it initates the transition. When
that happens, the virtio devices have already been used by the guest
firmware, bootloader, possibly another kernel that kexeced the "secure"
one, etc... 

So instead of running around saying NAK NAK NAK, please explain how we
can solve that differently.

Ben.