Re: [PATCH 1/2] PCI/P2PDMA: Allow P2PDMA in VMware guests on Intel hosts

[Logan Gunthorpe]

On 2026-10-09 9:40 a.m., 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. 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.
> 
> The P2PDMA host bridge check does not work in a VMware guest. Bypass it on
> Intel hosts under VMware, starting with Skylake-SP, the first Xeon
> Scalable generation. Xeon Scalable introduced UPI and is whitelisted for
> P2PDMA without restrictions. AMD Zen and newer are already covered by
> cpu_supports_p2pdma(), so the bypass is implemented for Intel hosts only.
> 
> 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).
> 
> Reviewed-by: Jim Chow <jim.chow@xxxxxxxxxxxx>
> Signed-off-by: Pavel Popov <pavel.e.popov@xxxxxxxxx>
> ---
>  drivers/pci/p2pdma.c | 20 ++++++++++++++++++++
>  1 file changed, 20 insertions(+)
> 
> diff --git a/drivers/pci/p2pdma.c b/drivers/pci/p2pdma.c
> index 9334eb314663..d96f44d0738a 100644
> --- a/drivers/pci/p2pdma.c
> +++ b/drivers/pci/p2pdma.c
> @@ -21,6 +21,12 @@
>  #include <linux/seq_buf.h>
>  #include <linux/xarray.h>
>  
> +#ifdef CONFIG_X86
> +#include <asm/cpu_device_id.h>
> +#include <asm/hypervisor.h>
> +#include <asm/intel-family.h>
> +#endif
> +
>  struct pci_p2pdma {
>  	struct gen_pool *pool;
>  	bool p2pmem_published;
> @@ -532,6 +538,19 @@ static bool cpu_supports_p2pdma(void)
>  	return false;
>  }
>  
> +static bool hypervisor_supports_p2pdma(void)
> +{
> +#ifdef CONFIG_X86
> +	/* Hypervisor hides host topology, allow p2pdma on Skylake and newer */
> +	if (boot_cpu_data.x86_vendor == X86_VENDOR_INTEL &&
> +	    boot_cpu_data.x86_vfm >= INTEL_SKYLAKE_X &&
> +	    hypervisor_is_type(X86_HYPER_VMWARE))
> +		return true;
> +#endif
> +
> +	return false;
> +}

The changes seem fine, but one minor nit from me. Can we avoid putting
the #ifdefs inside the function?

Something like:

#ifdef CONFIG_X86
static bool hypervisor_supports_p2pdma(void)
{
...
}
#else /* !CONFIG_X86 */
static bool hypervisor_supports_p2pdma(void)
{
	return false;
}
#endif /* CONFIG_X86 */


Other than that, the core concept seems fine to me:

Reviewed-by: Logan Gunthorpe <logang@xxxxxxxxxxxx>

Thanks!

Logan