Re: [PATCH] Whitelist AMD host bridge device(s) to enable P2P DMA

[Alex Deucher]

On Wed, Aug 11, 2021 at 4:50 PM Felix Kuehling <felix.kuehling@xxxxxxx> wrote:
>
>
> Am 2021-08-11 um 3:29 p.m. schrieb Alex Deucher:
> > On Wed, Aug 11, 2021 at 3:11 PM Ramesh Errabolu <Ramesh.Errabolu@xxxxxxx> wrote:
> >> Current implementation will disallow P2P DMA if the participating
> >> devices belong to different root complexes. Implementation allows
> >> this default behavior to be overridden for whitelisted devices. The
> >> patch adds an AMD host bridge to be whitelisted
> > Why do we need this?  cpu_supports_p2pdma() should return true for all
> > AMD Zen CPUs.
>
> This is part of our on-going work to get P2P support upstream. We want
> to use pci_p2pdma_distance_many to determine whether P2P is possible
> between a pair of devices. This whitelist is used in this function. This
> will affect the P2P links reported in the topology and it will be
> double-checked in the BO mapping function to ensure a peer mapping is legal.
>
> I think this change is a bit free of context at the moment, as we're
> still working on a few other loose ends for P2P support in our internal
> branch. I'm hoping we'll have a bigger patch series for upstreamable KFD
> P2P support ready in a few weeks. I also think we'll probably want to
> add a few more PCI IDs for other supported AMD root complexes.

We don't need to keep adding AMD root complexes.  You must be using an
older kernel or something.  All root complexes on all Zen platforms
support P2P DMA.  See:

commit dea286bb71baded7d2fb4f090e3b9fd2c1ccac58
Author: Logan Gunthorpe <logang@xxxxxxxxxxxx>
Date:   Wed Jul 29 17:18:44 2020 -0600

    PCI/P2PDMA: Allow P2PDMA on AMD Zen and newer CPUs

    Allow P2PDMA if the CPU vendor is AMD and family is 0x17 (Zen) or greater.

    [bhelgaas: commit log, simplify #if/#else/#endif]
    Link: https://lore.kernel.org/r/20200729231844.4653-1-logang@xxxxxxxxxxxx
    Signed-off-by: Logan Gunthorpe <logang@xxxxxxxxxxxx>
    Signed-off-by: Bjorn Helgaas <bhelgaas@xxxxxxxxxx>
    Reviewed-by: Alex Deucher <alexander.deucher@xxxxxxx>
    Cc: Christian König <christian.koenig@xxxxxxx>
    Cc: Huang Rui <ray.huang@xxxxxxx>

Alex


>
> Regards,
>   Felix
>
>
> >
> > Alex
> >
> >> Signed-off-by: Ramesh Errabolu <Ramesh.Errabolu@xxxxxxx>
> >> ---
> >>  drivers/pci/p2pdma.c | 2 ++
> >>  1 file changed, 2 insertions(+)
> >>
> >> diff --git a/drivers/pci/p2pdma.c b/drivers/pci/p2pdma.c
> >> index 196382630363..7003bb9faf23 100644
> >> --- a/drivers/pci/p2pdma.c
> >> +++ b/drivers/pci/p2pdma.c
> >> @@ -305,6 +305,8 @@ static const struct pci_p2pdma_whitelist_entry {
> >>         {PCI_VENDOR_ID_INTEL,   0x2032, 0},
> >>         {PCI_VENDOR_ID_INTEL,   0x2033, 0},
> >>         {PCI_VENDOR_ID_INTEL,   0x2020, 0},
> >> +       /* AMD Host Bridge Devices */
> >> +       {PCI_VENDOR_ID_AMD,     0x1480, 0},
> >>         {}
> >>  };
> >>
> >> --
> >> 2.31.1
> >>