linux-kernel - RE: [PATCH v3 1/1] PCI: Add translated request only flag for pci_enable

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Date:   Sat, 28 Jan 2023 07:52:07 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Bjorn Helgaas <helgaas@...nel.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>
CC:     Bjorn Helgaas <bhelgaas@...gle.com>,
        "Rodel, Jorg" <jroedel@...e.de>,
        "Matt Fagnani" <matt.fagnani@...l.net>,
        Christian König <christian.koenig@....com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Vasant Hegde <vasant.hegde@....com>,
        "Zhu, Tony" <tony.zhu@...el.com>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v3 1/1] PCI: Add translated request only flag for
 pci_enable_pasid()

> From: Bjorn Helgaas <helgaas@...nel.org>
> Sent: Saturday, January 28, 2023 1:31 AM
> 
> On Sat, Jan 14, 2023 at 03:34:20PM +0800, Lu Baolu wrote:
> 
> > @@ -353,12 +353,15 @@ void pci_pasid_init(struct pci_dev *pdev)
> >   * pci_enable_pasid - Enable the PASID capability
> >   * @pdev: PCI device structure
> >   * @features: Features to enable
> > + * @flags: device-specific flags
> > + *   - PCI_PASID_XLATED_REQ_ONLY: The PCI device always use translated
> type
> > + *                                for all PASID memory requests.
> 
> s/use/uses/
> 
> I guess PCI_PASID_XLATED_REQ_ONLY is something only the driver knows,
> right?  We can't deduce from architected config space that the device

Yes

> will produce PASID prefixes for every Memory Request, can we?
> 

No, we can't. PASID cap only indicates that the device is capable of
using PASID prefix, not a mandatory requirement on every memory
request.

Similar case is PRI. Having PRI enabled only means the device is
capable of handling I/O page faults, not the indicator that it can
100% tolerate fault in every memory access. That is the main reason
why vfio/iommufd can't simply skip pinning guest memory when
seeing PRI enabled (well, though PRI is not supported yet). It has to
be opted-in via a driver specific way e.g. a vfio variant driver.

Thanks
Kevin