linux-kernel - Re: [PATCH] PCI/ATS: Allow to enable ATS on VFs even if it is not enabled on PF

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Date:   Thu, 16 Feb 2023 09:46:51 +0200
From:   Leon Romanovsky <leon@...nel.org>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     Ganapatrao Kulkarni <gankulkarni@...amperecomputing.com>,
        linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
        bhelgaas@...gle.com, jean-philippe@...aro.org,
        darren@...amperecomputing.com, scott@...amperecomputing.com,
        Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Joerg Roedel <joro@...tes.org>,
        linux-arm-kernel@...ts.infradead.org, iommu@...ts.linux.dev
Subject: Re: [PATCH] PCI/ATS:  Allow to enable ATS on VFs even if it is not
 enabled on PF

On Thu, Feb 16, 2023 at 09:26:15AM +0200, Leon Romanovsky wrote:
> On Wed, Feb 15, 2023 at 02:57:26PM -0600, Bjorn Helgaas wrote:
> > [+cc Will, Robin, Joerg for arm-smmu-v3 page size question]
> > 
> > On Sun, Feb 12, 2023 at 08:14:48PM +0200, Leon Romanovsky wrote:
> > > On Wed, Feb 08, 2023 at 10:43:21AM -0800, Ganapatrao Kulkarni wrote:
> > > > As per PCIe specification(section 10.5), If a VF implements an
> > > > ATS capability, its associated PF must implement an ATS capability.
> > > > The ATS Capabilities in VFs and their associated PFs are permitted to
> > > > be enabled independently.
> > > > Also, it states that the Smallest Translation Unit (STU) for VFs must be
> > > > hardwired to Zero and the associated PF's value applies to VFs STU.
> > > > 
> > > > The current code allows to enable ATS on VFs only if it is already
> > > > enabled on associated PF, which is not necessary as per the specification.
> > > > 
> > > > It is only required to have valid STU programmed on PF to enable
> > > > ATS on VFs. Adding code to write the first VFs STU to a PF's STU
> > > > when PFs ATS is not enabled.
> > >
> > > Can you please add here quotes from the spec and its version? I don't see
> > > anything like this in my version of PCIe specification.
> > 
> > See PCIe r6.0, sec 10.5.1.
> 
> Awesome, I have old versions.

OK, where should I read about this sentence?

"It is only required to have valid STU programmed on PF to enable
 ATS on VFs. Adding code to write the first VFs STU to a PF's STU
 when PFs ATS is not enabled."

>From spec:
"Smallest Translation Unit (STU) - This value indicates to the Function the minimum number of
4096-byte blocks that is indicated in a Translation Completions or Invalidate Requests. This is a power of
2 multiplier and the number of blocks is 2STU. A value of 0 0000b indicates one block and a value of
1 1111b indicates 231 blocks (or 8 TB total)

For VFs, this field must be hardwired to Zero. The associated PF's value applies.
Default value is 0 0000b"

And enable bit doesn't have any sentence about STU.

Thanks