Re: [PATCH v3 8/8] iommu/arm-smmu-v3: Make default domain type of HiSilicon PTT device to identity

[Robin Murphy]

On 2022-02-15 13:42, Will Deacon wrote:
> On Tue, Feb 15, 2022 at 01:30:26PM +0000, Robin Murphy wrote:
> > On 2022-02-15 13:00, Will Deacon wrote:
> > > On Mon, Feb 14, 2022 at 08:55:20PM +0800, Yicong Yang wrote:
> > > > On 2022/1/24 21:11, Yicong Yang wrote:
> > > > > The DMA of HiSilicon PTT device can only work with identical
> > > > > mapping. So add a quirk for the device to force the domain
> > > > > passthrough.
> > > > >
> > > > > Signed-off-by: Yicong Yang <yangyicong@xxxxxxxxxxxxx>
> > > > > ---
> > > > >    drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c | 16 ++++++++++++++++
> > > > >    1 file changed, 16 insertions(+)
> > > > >
> > > > > diff --git a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> > > > > index 6dc6d8b6b368..6f67a2b1dd27 100644
> > > > > --- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> > > > > +++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> > > > > @@ -2838,6 +2838,21 @@ static int arm_smmu_dev_disable_feature(struct device *dev,
> > > > >    	}
> > > > >    }
> > > > > +#define IS_HISI_PTT_DEVICE(pdev)	((pdev)->vendor == PCI_VENDOR_ID_HUAWEI && \
> > > > > +					 (pdev)->device == 0xa12e)
> > > > > +
> > > > > +static int arm_smmu_def_domain_type(struct device *dev)
> > > > > +{
> > > > > +	if (dev_is_pci(dev)) {
> > > > > +		struct pci_dev *pdev = to_pci_dev(dev);
> > > > > +
> > > > > +		if (IS_HISI_PTT_DEVICE(pdev))
> > > > > +			return IOMMU_DOMAIN_IDENTITY;
> > > > > +	}
> > > > > +
> > > > > +	return 0;
> > > > > +}
> > > > > +
> > > > >    static struct iommu_ops arm_smmu_ops = {
> > > > >    	.capable		= arm_smmu_capable,
> > > > >    	.domain_alloc		= arm_smmu_domain_alloc,
> > > > > @@ -2863,6 +2878,7 @@ static struct iommu_ops arm_smmu_ops = {
> > > > >    	.sva_unbind		= arm_smmu_sva_unbind,
> > > > >    	.sva_get_pasid		= arm_smmu_sva_get_pasid,
> > > > >    	.page_response		= arm_smmu_page_response,
> > > > > +	.def_domain_type	= arm_smmu_def_domain_type,
> > > > >    	.pgsize_bitmap		= -1UL, /* Restricted during device attach */
> > > > >    	.owner			= THIS_MODULE,
> > > > >    };
> > > >
> > > > Is this quirk ok with the SMMU v3 driver? Just want to confirm that I'm on the
> > > > right way to dealing with the issue of our device.
> > >
> > > I don't think the quirk should be in the SMMUv3 driver. Assumedly, you would
> > > have the exact same problem if you stuck the PTT device behind a different
> > > type of IOMMU, and so the quirk should be handled by a higher level of the
> > > stack.
> >
> > Conceptually, yes, but I'm inclined to be pragmatic here. Default domain
> > quirks could only move out as far as the other end of the call from
> > iommu_get_def_domain_type() - it's not like we could rely on some flag in a
> > driver which may not even be loaded yet, let alone matched to the device.
> > And even then there's an equal and opposite argument for why the core code
> > should have to maintain a list of platform-specific quirks rather than code
> > specific to the relevant platforms. The fact is that a HiSilicon RCiEP is
> > not going to end up behind anything other than a HiSilicon IOMMU, and if
> > those ever stop being SMMUv3 *and* such a quirk still exists we can worry
> > about it then.
>
> Perhaps, but you know that by adding this hook it's only a matter of time
> before we get random compatible string matches in there, so I'd rather keep
> the flood gates closed as long as we can.
>
> Given that this is a PCI device, why can't we have a PCI quirk for devices
> which require an identity mapping and then handle that in the IOMMU core?

Oh, don't think I *like* having quirks in the driver, it just seems like 
the least-worst choice from a bad bunch. All of the default domain 
quirks so far (including this one) exist for integrated devices and/or 
dodgy firmware setups such that they are platform-specific, so there is 
no technical reason for trying to split *some* of them off into a 
generic mechanism when the driver-based platform-specific mechanism 
still needs to exist anyway (some of them do depend on driver state as 
well).

Feel free to test the waters with a patch punting 
qcom_smmu_def_domain_type() to core code, but I think you'll struggle to 
find a reason to give in the commit message other than "I don't like it".

> > Ugly as it is, this is the status quo. I don't recall anyone ever arguing
> > that the equivalent quirks for Intel integrated graphics should be made
> > generic ;)
>
> I don't know anything about Intel integrated graphics. Have they solved this
> problem in a better way, or could they equally make use of a generic quirk?

See intel-iommu's device_def_domain_type() implementation. The shape of 
it may seem quite familiar...

Robin.