Re: [PATCH] intel-iommu: don't disable ATS for device without page aligned request

From: Jason Wang <jasowang@redhat.com>
To: Ashok Raj <ashok.raj@intel.com>
Cc: mst@redhat.com, linux-kernel@vger.kernel.org,
	Keith Busch <keith.busch@intel.com>,
	eperezma@redhat.com, iommu@lists.linux-foundation.org,
	stable@vger.kernel.org, dwmw2@infradead.org
Subject: Re: [PATCH] intel-iommu: don't disable ATS for device without page aligned request
Date: Wed, 9 Sep 2020 22:17:35 -0400 (EDT)	[thread overview]
Message-ID: <491540137.16465450.1599704255365.JavaMail.zimbra@redhat.com> (raw)
In-Reply-To: <20200909171056.GF104641@otc-nc-03>

----- Original Message -----
> Hi Jason
> 
> On Wed, Sep 09, 2020 at 04:34:32PM +0800, Jason Wang wrote:
> > Commit 61363c1474b1 ("iommu/vt-d: Enable ATS only if the device uses
> > page aligned address.") disables ATS for device that can do unaligned
> > page request.
> 
> Did you take a look at the PCI specification?
> Page Aligned Request is in the ATS capability Register.
> 
> ATS Capability Register (Offset 0x04h)
> 
> bit (5):
> Page Aligned Request - If Set, indicates the Untranslated address is always
> aligned to 4096 byte boundary. Setting this field is recommended. This
> field permits software to distinguish between implemntations compatible
> with this specification and those compatible with an earlier version of
> this specification in which a Requester was permitted to supply anything in
> bits [11:2].

Yes, my understanding is that this is optional not mandatory.

> 
> > 
> > This looks wrong, since the commit log said it's because the page
> > request descriptor doesn't support reporting unaligned request.
> 
> I don't think you can change the definition from ATS to PRI. Both are
> orthogonal feature.

I may miss something, here's my understanding is that:

- page request descriptor will only be used when PRS is enabled
- ATS spec allows unaligned request

So any reason for disabling ATS for unaligned request even if PRS is
not enabled?

> 
> > 
> > A victim is Qemu's virtio-pci which doesn't advertise the page aligned
> > address. Fixing by disable PRI instead of ATS if device doesn't have
> > page aligned request.
> 
> This is a requirement for the Intel IOMMU's.
> 
> You say virtio, so is it all emulated device or you talking about some
> hardware that implemented virtio-pci compliant hw? If you are sure the
> device actually does comply with the requirement, but just not enumerating
> the capability, you can maybe work a quirk to overcome that?

So far only emulated devices. But we are helping some vendor to
implement virtio hardware so  we need to understand the connection
between ATS alignment and page request descriptor.

> 
> Now PRI also has an alignment requirement, and Intel IOMMU's requires that
> as well. If your device supports SRIOV as well, PASID and PRI are
> enumerated just on the PF and not the VF. You might want to pay attension
> to that. We are still working on a solution for that problem.

Thanks for the reminding, but it looks to me according to the ATS
spec, all PRI message is 4096 byte aligned? E.g lower bites were used
for group index etc.

Thanks

> 
> I don't think this is the right fix for your problem.
> 
> > 
> > Cc: stable@vger.kernel.org
> > Cc: Ashok Raj <ashok.raj@intel.com>
> > Cc: Jacob Pan <jacob.jun.pan@linux.intel.com>
> > Cc: Keith Busch <keith.busch@intel.com>
> > Cc: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
> > Signed-off-by: Jason Wang <jasowang@redhat.com>
> > ---
> >  drivers/iommu/intel/iommu.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> 
> 

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