Re: [PATCH v3] PCI/P2PDMA: Allow P2PDMA on Intel Xeon server CPUs

[Bjorn Helgaas <helgaas@kernel.org>]

On Tue, Oct 20, 2026 at 07:14:47PM +0300, Leon Romanovsky wrote:
> On Tue, Oct 20, 2026 at 08:13:05AM -0700, Pavel Popov wrote:
> > Peer-to-Peer DMA (P2PDMA) between two devices requires either a shared
> > upstream bridge or a host bridge matching pci_p2pdma_whitelist[]. In
> > VMware guests, passthrough devices appear directly on virtual root
> > buses without host bridge devices. In Hyper-V guests, each passthrough
> > device appears in a PCI domain of its own without a host bridge
> > device. Consequently, P2PDMA checks evaluate endpoint Device IDs
> > against the host bridge whitelist, causing P2PDMA requests to fail
> > regardless of host support.
> > 
> > This affects discrete devices such as GPUs, NICs and NVMe drives.
> > 
> > Add an explicit list of the Intel Xeon server generations from
> > Skylake-SP to Diamond Rapids to cpu_supports_p2pdma(). These are the
> > CPUs whose host bridges are in pci_p2pdma_whitelist[] without
> > restrictions. AMD Zen and newer are already covered by
> > cpu_supports_p2pdma(), so only Intel entries are needed.
> > 
> > Tested with the Intel Level Zero P2P benchmark (ze_peer) on Arc Pro B60
> > cards in a Linux guest on VMware ESXi 9.0 (Emerald Rapids host).
> > 
> > Signed-off-by: Pavel Popov <pavel.e.popov@intel.com>
> > ---
> > Changes in v3:
> > - Drop the hypervisor check and move the Xeon list into
> >   cpu_supports_p2pdma(), next to the AMD check, as suggested by Logan
> >   Gunthorpe and Leon Romanovsky [1]
> > - Fold the VMware and Hyper-V patches into one, as the check no longer
> >   depends on the hypervisor
> > 
> > Changes in v2:
> > - Replace the Skylake-and-newer threshold with an explicit list of Intel
> >   Xeon server generations, from Skylake-SP to Diamond Rapids (Logan
> >   Gunthorpe)
> > - Move the CONFIG_X86 #ifdef out of hypervisor_supports_p2pdma()
> >   (Logan Gunthorpe)
> > - Rebased on current mainline
> > 
> > [1] https://lore.kernel.org/r/20261016153147.GC219304@unreal
> > 
> > v2: https://lore.kernel.org/r/20261016223402.19991-1-pavel.e.popov@intel.com
> > v1: https://lore.kernel.org/r/20261009154034.82321-1-pavel.e.popov@intel.com
> > 
> >  drivers/pci/p2pdma.c | 26 ++++++++++++++++++++++++++
> >  1 file changed, 26 insertions(+)
> > 
> > diff --git a/drivers/pci/p2pdma.c b/drivers/pci/p2pdma.c
> > index 9334eb314663..2650d605d87d 100644
> > --- a/drivers/pci/p2pdma.c
> > +++ b/drivers/pci/p2pdma.c
> > @@ -21,6 +21,11 @@
> >  #include <linux/seq_buf.h>
> >  #include <linux/xarray.h>
> >  
> > +#ifdef CONFIG_X86
> > +#include <asm/cpu_device_id.h>
> > +#include <asm/intel-family.h>
> > +#endif
> > +
> >  struct pci_p2pdma {
> >  	struct gen_pool *pool;
> >  	bool p2pmem_published;
> > @@ -519,6 +524,23 @@ static void seq_buf_print_bus_devfn(struct seq_buf *buf, struct pci_dev *pdev)
> >  	seq_buf_printf(buf, "%s;", pci_name(pdev));
> >  }
> >  
> > +#ifdef CONFIG_X86
> 
> Do we really need all these #ifdefs? I'm pretty sure they're not
> needed for the includes.

Apparently we do.  I experimentally removed them and cross-compiled
for arm64:

  drivers/pci/p2pdma.c:24:10: fatal error: asm/cpu_device_id.h: No such file or directory
     24 | #include <asm/cpu_device_id.h>
	|          ^~~~~~~~~~~~~~~~~~~~~