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

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Message-ID: <20180809054013.GA20507@infradead.org>
Date:   Wed, 8 Aug 2018 22:40:13 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        Christoph Hellwig <hch@...radead.org>,
        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 Thu, Aug 09, 2018 at 08:13:32AM +1000, Benjamin Herrenschmidt wrote:
> > > -	if (xen_domain())
> > > +	if (xen_domain() || pseries_secure_vm())
> > > 		return true;
> > 
> > I don't think it's pseries specific actually. E.g. I suspect AMD SEV
> > might benefit from the same kind of hack.
> 
> As long as they can provide the same guarantee that the DMA ops are
> completely equivalent between virtio and other PCI devices, at least on
> the same bus, ie, we don't have to go hack special DMA ops.
> 
> I think the latter is really what Christoph wants to avoid for good
> reasons.

Yes.  I also generally want to avoid too much arch specific magic.

FYI, I'm off to a week-long vacation today, don't expect quick replies.