linux-kernel - Re: [PATCH] PCI/P2PDMA: Add AMD Zen 2 root complex to the list of allowed bridges

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Message-ID: <CADnq5_MCPTyxG31guPFL-uvs7HisGxwO5KpALnufc=Bj4MfYCw@mail.gmail.com>
Date:   Fri, 24 Jul 2020 12:07:04 -0400
From:   Alex Deucher <alexdeucher@...il.com>
To:     Logan Gunthorpe <logang@...tatee.com>
Cc:     Bjorn Helgaas <helgaas@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux PCI <linux-pci@...r.kernel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Christian König <christian.koenig@....com>,
        Huang Rui <ray.huang@....com>,
        Andrew Maier <andrew.maier@...eticom.com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] PCI/P2PDMA: Add AMD Zen 2 root complex to the list of
 allowed bridges

On Thu, Jul 23, 2020 at 4:18 PM Alex Deucher <alexdeucher@...il.com> wrote:
>
> On Thu, Jul 23, 2020 at 4:11 PM Logan Gunthorpe <logang@...tatee.com> wrote:
> >
> >
> >
> > On 2020-07-23 1:57 p.m., Bjorn Helgaas wrote:
> > > [+cc Andrew, Armen, hpa]
> > >
> > > On Thu, Jul 23, 2020 at 02:01:17PM -0400, Alex Deucher wrote:
> > >> On Thu, Jul 23, 2020 at 1:43 PM Logan Gunthorpe <logang@...tatee.com> wrote:
> > >>>
> > >>> The AMD Zen 2 root complex (Starship/Matisse) was tested for P2PDMA
> > >>> transactions between root ports and found to work. Therefore add it
> > >>> to the list.
> > >>>
> > >>> Signed-off-by: Logan Gunthorpe <logang@...tatee.com>
> > >>> Cc: Bjorn Helgaas <bhelgaas@...gle.com>
> > >>> Cc: Christian König <christian.koenig@....com>
> > >>> Cc: Huang Rui <ray.huang@....com>
> > >>> Cc: Alex Deucher <alexdeucher@...il.com>
> > >>
> > >> Starting with Zen, all AMD platforms support P2P for reads and writes.
> > >
> > > What's the plan for getting out of the cycle of "update this list for
> > > every new chip"?  Any new _DSMs planned, for instance?
> >
> > Well there was an effort to add capabilities in the PCI spec to describe
> > this but, as far as I know, they never got anywhere, and hardware still
> > doesn't self describe with this.
> >
> > > A continuous trickle of updates like this is not really appealing.  So
> > > far we have:
> > >
> > >   7d5b10fcb81e ("PCI/P2PDMA: Add AMD Zen Raven and Renoir Root Ports to whitelist")
> > >   7b94b53db34f ("PCI/P2PDMA: Add Intel Sky Lake-E Root Ports B, C, D to the whitelist")
> > >   bc123a515cb7 ("PCI/P2PDMA: Add Intel SkyLake-E to the whitelist")
> > >   494d63b0d5d0 ("PCI/P2PDMA: Whitelist some Intel host bridges")
> > >   0f97da831026 ("PCI/P2PDMA: Allow P2P DMA between any devices under AMD ZEN Root Complex")
> > >
> > > And that's just from the last year, not including this patch.
> >
> > Yes, it's not ideal. But most of these are adding old devices as people
> > test and care about running on those platforms -- a lot of this is
> > bootstrapping the list. I'd expect this to slow down a bit as by now we
> > have hopefully got a lot of the existing platforms people care about.
> > But we'd still probably expect to be adding a new Intel and AMD devices
> > about once a year as they produce new hardware designs.
> >
> > Unless, the Intel and AMD folks know of a way to detect this, or even to
> > query if a root complex is newer than a certain generation, I'm not sure
> > what else we can do here.
>
> I started a thread internally to see if I can find a way.  FWIW,
> pre-ZEN parts also support p2p DMA, but only for writes.  If I can get
> a definitive list, maybe we could switch to a blacklist for the old
> ones?

After talking with a few people internally, for AMD chips, it would
probably be easiest to just whitelist based on the CPU family id for
zen and newer (e.g., >= 0x17).

Alex

>
> Alex
>
> >
> > Logan