Re: [PATCH v19 09/18] vfio/pci: Hide TPH capability when TPH is unsupported

[Alex Williamson <alex@shazbot.org>]

On Thu, 2 Jul 2026 20:42:15 +0800
Chengwen Feng <fengchengwen@huawei.com> wrote:

> Check the device negotiated TPH support status before parsing the TPH
> extended capability. Return zero length to hide the capability from
> userspace if TPH is disabled during topology negotiation.

This doesn't match the proposal and introduces an ABI incompatibility
to the user.  A device that includes a TPH capability and supports
migration would break if the source and target systems disagree about
whether the capability is present, even if read-only.

The proposal did not suggest hiding the TPH capability, instead it used
SET on the device feature ioctl as the opt-in mechanism to introduce
virtualization of the TPH capability and control registers.  Nak to
this change.  Thanks,

Alex

> Signed-off-by: Chengwen Feng <fengchengwen@huawei.com>
> ---
>  drivers/vfio/pci/vfio_pci_config.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/vfio/pci/vfio_pci_config.c b/drivers/vfio/pci/vfio_pci_config.c
> index a10ed733f0e3..5c6ab172df6c 100644
> --- a/drivers/vfio/pci/vfio_pci_config.c
> +++ b/drivers/vfio/pci/vfio_pci_config.c
> @@ -22,6 +22,7 @@
>  
>  #include <linux/fs.h>
>  #include <linux/pci.h>
> +#include <linux/pci-tph.h>
>  #include <linux/uaccess.h>
>  #include <linux/vfio.h>
>  #include <linux/slab.h>
> @@ -1450,6 +1451,8 @@ static int vfio_ext_cap_len(struct vfio_pci_core_device *vdev, u16 ecap, u16 epo
>  		byte &= PCI_DPA_CAP_SUBSTATE_MASK;
>  		return PCI_DPA_BASE_SIZEOF + byte + 1;
>  	case PCI_EXT_CAP_ID_TPH:
> +		if (!pcie_tph_supported(pdev, false))
> +			return 0;
>  		ret = pci_read_config_dword(pdev, epos + PCI_TPH_CAP, &dword);
>  		if (ret)
>  			return pcibios_err_to_errno(ret);