Re: [PATCH v19 17/18] vfio/pci: Reset hardware TPH state on device enable/disable

[Alex Williamson <alex@shazbot.org>]

On Wed, 15 Jul 2026 11:09:28 +0800
fengchengwen <fengchengwen@huawei.com> wrote:
> On 7/9/2026 7:29 AM, Alex Williamson wrote:
> > On Thu, 2 Jul 2026 20:42:23 +0800
> > Chengwen Feng <fengchengwen@huawei.com> wrote:
> >   
> >> VFIO allows user space to enable and modify PCI TPH state, but device
> >> reset only clears hardware TPH registers, while the software TPH state
> >> maintained in pci/tph.c remains stale.  
> > 
> > It's saved and restored, why is it "stale"?  It seems more that the
> > interface simply doesn't meet our use case.
> > 
> > What about resets through vfio-pci interfaces, should TPH be
> > automatically disabled?  Thanks,  
> 
> I initially added TPH disable logic into VFIO reset paths in v20 to make
> all reset scenarios consistent, but after reviewing Sashiko's feedback
> and re-checking PCI core state management, I agree we do NOT need extra
> pcie_disable_tph() calls in VFIO reset ioctl handlers.
> 
> VFIO reset ioctls eventually trigger the PCI state save/restore flow,
> which invokes pci_save_tph_state() and pci_restore_tph_state() to handle
> TPH capability and control registers properly. TPH configuration state
> will be restored to the saved value automatically after reset.
> 
> One important exception is the Steering Tag table when located in MSI-X
> vector space (PCI_TPH_LOC_MSIX). The pci_save_tph_state() helper only
> saves TPH capability registers; it does not back up ST entries stored in
> MSI-X table entries. After any PCI reset, those ST values are lost along
> with the rest of the MSI-X table and are not restored automatically.

False.  It's not restored as a separate TPH operation, the ST
upper/lower is stored in the MSI-X vector control register, which is
restored from kernel state (without a separate save-state function)
via:

__pci_restore_msix_state()
	pci_msix_write_vector_ctrl(entry, entry->pci.msix_ctrl);
 
> I will add explicit documentation to the VFIO_DEVICE_FEATURE_TPH_ST uAPI
> comments stating that userspace must re-program all ST table entries via
> this feature after any device reset, as ST entries are not preserved
> across PCI resets (especially for MSI-X backed ST tables).

Only DS is not saved and restored.

Alex