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

[fengchengwen <fengchengwen@huawei.com>]

On 7/9/2026 7:32 AM, Alex Williamson wrote:
> 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,

Got it, will drop this commit in v20

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);
>