RE: [PATCH] iommu/vt-d: Add a fix for devices need extra dtlb flush

["Tian, Kevin" <kevin.tian@intel.com>]

> From: Baolu Lu <baolu.lu@linux.intel.com>
> Sent: Wednesday, November 23, 2022 1:04 PM
> 
> On 2022/11/23 9:02, Tian, Kevin wrote:
> >> From: Robin Murphy <robin.murphy@arm.com>
> >> Sent: Wednesday, November 23, 2022 1:49 AM
> >>
> >>> +
> >>> +/* Impacted QAT device IDs ranging from 0x4940 to 0x4943 */
> >>> +#define BUGGY_QAT_DEVID_MASK 0x494c
> >>> +static bool dev_needs_extra_dtlb_flush(struct pci_dev *pdev)
> >>> +{
> >>> +	if (pdev->vendor != PCI_VENDOR_ID_INTEL)
> >>> +		return false;
> >>> +
> >>> +	if ((pdev->device & 0xfffc) != BUGGY_QAT_DEVID_MASK)
> >>> +		return false;
> >>> +
> >>> +	if (risky_device(pdev))
> >>> +		return false;
> >>
> >> Hmm, I'm not sure that that makes much sense to me - what privilege can
> >> the device gain from being told to invalidate things twice? Why would we
> >> want to implicitly *allow* a device to potentially keep using a stale
> >> translation if for some bizarre reason firmware has marked it as
> >> external, surely that's worse?
> 
>  From the perspective of IOMMU, any quirk is only applicable to trusted
> devices. If the IOMMU driver detects that a quirk is being applied to an
> untrusted device, it is already buggy or malicious. The IOMMU driver
> should let the users know by:
> 
> 	pci_info(pdev,
> 		 "Skipping IOMMU quirk for dev [%04X:%04X] on untrusted
> PCI link\n",
> 		 pdev->vendor, pdev->device);
> 	pci_info(pdev, "Please check with your BIOS/Platform vendor about
> this\n");
> 
> and stop applying any quirk.
> 

A quirk usually relaxes something then you want it only on trusted devices.

but the quirk in this patch is trying to fix a vulnerability. In concept it's
weird to skip it on untrusted devices. This iiuc was the part causing confusion
to Robin.