From: Jean-Philippe Brucker <jean-philippe@linaro.org>
To: "Raj, Ashok" <ashok.raj@intel.com>
Cc: dwmw2@infradead.org, baolu.lu@linux.intel.com, joro@8bytes.org,
zhangfei.gao@linaro.org, wangzhou1@hisilicon.com, arnd@arndb.de,
gregkh@linuxfoundation.org, iommu@lists.linux-foundation.org,
linux-kernel@vger.kernel.org,
linux-accelerators@lists.ozlabs.org, kevin.tian@intel.com,
jacob.jun.pan@linux.intel.com, linux-pci@vger.kernel.org, "Lu,
Baolu" <baolu.lu@intel.com>,
Jacon Jun Pan <jacob.jun.pan@intel.com>
Subject: Re: [RFC PATCH 0/2] iommu: Avoid unnecessary PRI queue flushes
Date: Fri, 23 Oct 2020 15:34:23 +0200 [thread overview]
Message-ID: <20201023133423.GF2265982@myrica> (raw)
In-Reply-To: <20201019211608.GA79633@otc-nc-03>
On Mon, Oct 19, 2020 at 02:16:08PM -0700, Raj, Ashok wrote:
> Hi Jean
>
> On Mon, Oct 19, 2020 at 04:08:24PM +0200, Jean-Philippe Brucker wrote:
> > On Sat, Oct 17, 2020 at 04:25:25AM -0700, Raj, Ashok wrote:
> > > > For devices that *don't* use a stop marker, the PCIe spec says (10.4.1.2):
> > > >
> > > > To stop [using a PASID] without using a Stop Marker Message, the
> > > > function shall:
> > > > 1. Stop queueing new Page Request Messages for this PASID.
> > >
> > > The device driver would need to tell stop sending any new PR's.
> > >
> > > > 2. Finish transmitting any multi-page Page Request Messages for this
> > > > PASID (i.e. send the Page Request Message with the L bit Set).
> > > > 3. Wait for PRG Response Messages associated any outstanding Page
> > > > Request Messages for the PASID.
> > > >
> > > > So they have to flush their PR themselves. And since the device driver
> > > > completes this sequence before calling unbind(), then there shouldn't be
> > > > any oustanding PR for the PASID, and unbind() doesn't need to flush,
> > > > right?
> > >
> > > I can see how the device can complete #2,3 above. But the device driver
> > > isn't the one managing page-responses right. So in order for the device to
> > > know the above sequence is complete, it would need to get some assist from
> > > IOMMU driver?
> >
> > No the device driver just waits for the device to indicate that it has
> > completed the sequence. That's what the magic stop-PASID mechanism
> > described by PCIe does. In 6.20.1 "Managing PASID TLP Prefix Usage" it
> > says:
>
> The goal is we do this when the device is in a messup up state. So we can't
> take for granted the device is properly operating which is why we are going
> to wack the device with a flr().
>
> The only thing that's supposed to work without a brain freeze is the
> invalidation logic. Spec requires devices to respond to invalidations even when
> they are in the process of flr().
>
> So when IOMMU does an invalidation wait with a Page-Drain, IOMMU waits till
> the response for that arrives before completing the descriptor. Due to
> the posted semantics it will ensure any PR's issued and in the fabric are flushed
> out to memory.
>
> I suppose you can wait for the device to vouch for all responses, but that
> is assuming the device is still functioning properly. Given that we use it
> in two places,
>
> * Reclaiming a PASID - only during a tear down sequence, skipping it
> doesn't really buy us much.
Yes I was only wondering about normal PASID reclaim operations, in
unbind(). Agreed that for FLR we need to properly clean the queue, though
I do need to do more thinking about this.
Anyway, having a full priq drain in unbind() isn't harmful, just
unnecessary delay in my opinion. I'll drop these patches for now but
thanks for the discussion.
Thanks,
Jean
> * During FLR this can't be skipped anyway due to the above sequence
> requirement.
>
> >
> > "A Function must have a mechanism to request that it gracefully stop using
> > a specific PASID. This mechanism is device specific but must satisfy the
> > following rules:
> > [...]
> > * When the stop request mechanism indicates completion, the Function has:
> > [...]
> > * Complied with additional rules described in Address Translation
> > Services (Chapter 10 [10.4.1.2 quoted above]) if Address Translations
> > or Page Requests were issued on the behalf of this PASID."
> >
> > So after the device driver initiates this mechanism in the device, the
> > device must be able to indicate completion of the mechanism, which
> > includes completing all in-flight Page Requests. At that point the device
> > driver can call unbind() knowing there is no pending PR for this PASID.
> >
>
> Cheers,
> Ashok
prev parent reply other threads:[~2020-10-23 13:34 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-10-15 9:00 [RFC PATCH 0/2] iommu: Avoid unnecessary PRI queue flushes Jean-Philippe Brucker
2020-10-15 9:00 ` [RFC PATCH 1/2] iommu: Add flags to sva_unbind() Jean-Philippe Brucker
2020-10-15 9:00 ` [RFC PATCH 2/2] iommu: Add IOMMU_UNBIND_FAULT_PENDING flag Jean-Philippe Brucker
2020-10-16 7:07 ` Christoph Hellwig
2020-10-15 18:22 ` [RFC PATCH 0/2] iommu: Avoid unnecessary PRI queue flushes Raj, Ashok
2020-10-16 7:59 ` Jean-Philippe Brucker
2020-10-17 11:25 ` Raj, Ashok
2020-10-19 14:08 ` Jean-Philippe Brucker
2020-10-19 18:33 ` Jacob Pan
2020-10-23 13:30 ` Jean-Philippe Brucker
2020-10-19 21:16 ` Raj, Ashok
2020-10-23 13:34 ` Jean-Philippe Brucker [this message]
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