Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Wed, 1 Jul 2020 16:47:29 -0400
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     Stefano Stabellini <sstabellini@kernel.org>
Cc:     Christoph Hellwig <hch@infradead.org>, jgross@suse.com,
        Peng Fan <peng.fan@nxp.com>, x86@kernel.org,
        konrad.wilk@oracle.com, jasowang@redhat.com,
        linux-kernel@vger.kernel.org,
        virtualization@lists.linux-foundation.org,
        iommu@lists.linux-foundation.org, linux-imx@nxp.com,
        xen-devel@lists.xenproject.org, boris.ostrovsky@oracle.com,
        linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH] xen: introduce xen_vring_use_dma
Message-ID: <20200701164501-mutt-send-email-mst@kernel.org>
References: <20200624050355-mutt-send-email-mst@kernel.org>
 <alpine.DEB.2.21.2006241047010.8121@sstabellini-ThinkPad-T480s>
 <20200624163940-mutt-send-email-mst@kernel.org>
 <alpine.DEB.2.21.2006241351430.8121@sstabellini-ThinkPad-T480s>
 <20200624181026-mutt-send-email-mst@kernel.org>
 <alpine.DEB.2.21.2006251014230.8121@sstabellini-ThinkPad-T480s>
 <20200626110629-mutt-send-email-mst@kernel.org>
 <alpine.DEB.2.21.2006291621300.8121@sstabellini-ThinkPad-T480s>
 <20200701133456.GA23888@infradead.org>
 <alpine.DEB.2.21.2007011020320.8121@sstabellini-ThinkPad-T480s>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <alpine.DEB.2.21.2007011020320.8121@sstabellini-ThinkPad-T480s>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Wed, Jul 01, 2020 at 10:34:53AM -0700, Stefano Stabellini wrote:
> On Wed, 1 Jul 2020, Christoph Hellwig wrote:
> > On Mon, Jun 29, 2020 at 04:46:09PM -0700, Stefano Stabellini wrote:
> > > > I could imagine some future Xen hosts setting a flag somewhere in the
> > > > platform capability saying "no xen specific flag, rely on
> > > > "VIRTIO_F_ACCESS_PLATFORM". Then you set that accordingly in QEMU.
> > > > How about that?
> > > 
> > > Yes, that would be fine and there is no problem implementing something
> > > like that when we get virtio support in Xen. Today there are still no
> > > virtio interfaces provided by Xen to ARM guests (no virtio-block/net,
> > > etc.)
> > > 
> > > In fact, in both cases we are discussing virtio is *not* provided by
> > > Xen; it is a firmware interface to something entirely different:
> > > 
> > > 1) virtio is used to talk to a remote AMP processor (RPMesg)
> > > 2) virtio is used to talk to a secure-world firmware/OS (Trusty)
> > >
> > > VIRTIO_F_ACCESS_PLATFORM is not set by Xen in these cases but by RPMesg
> > > and by Trusty respectively. I don't know if Trusty should or should not
> > > set VIRTIO_F_ACCESS_PLATFORM, but I think Linux should still work
> > > without issues.
> > > 
> > 
> > Any virtio implementation that is not in control of the memory map
> > (aka not the hypervisor) absolutely must set VIRTIO_F_ACCESS_PLATFORM,
> > else it is completely broken.
> 
> Lots of broken virtio implementations out there it would seem :-(

Not really, most of virtio implementations are in full control of
memory, being part of the hypervisor.

> 
> > > The xen_domain() check in Linux makes it so that vring_use_dma_api
> > > returns the opposite value on native Linux compared to Linux as Xen/ARM
> > > DomU by "accident". By "accident" because there is no architectural
> > > reason why Linux Xen/ARM DomU should behave differently compared to
> > > native Linux in this regard.
> > > 
> > > I hope that now it is clearer why I think the if (xen_domain()) check
> > > needs to be improved anyway, even if we fix generic dma_ops with virtio
> > > interfaces missing VIRTIO_F_ACCESS_PLATFORM.
> > 
> > IMHO that Xen quirk should never have been added in this form..
> 
> Would you be in favor of a more flexible check along the lines of the
> one proposed in the patch that started this thread:
> 
>     if (xen_vring_use_dma())
>             return true;
> 
> 
> xen_vring_use_dma would be implemented so that it returns true when
> xen_swiotlb is required and false otherwise.

I'll need to think about it. Sounds reasonable on the surface ...

-- 
MST