linux-kernel - Re: [PATCH] PCI: Blacklist AMD Stoney GPU devices for ATS

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Date:   Wed, 29 Mar 2017 00:26:27 +0200
From:   'Joerg Roedel' <jroedel@...e.de>
To:     "Deucher, Alexander" <Alexander.Deucher@....com>
Cc:     'Joerg Roedel' <joro@...tes.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Daniel Drake <drake@...lessm.com>,
        "Nath, Arindam" <Arindam.Nath@....com>
Subject: Re: [PATCH] PCI: Blacklist AMD Stoney GPU devices for ATS

On Tue, Mar 28, 2017 at 09:13:23PM +0000, Deucher, Alexander wrote:
> If I understand Arindam's patch correctly, it only flushes TLB entries
> for domains in the flush queue whereas the previous behavior was to
> flush all domains.  If there was no TLB flush in the queue for that
> domain, could flushing it cause a problem?

No, that can't cause a problem. An io/tlb flush for the device is just a
message that the device should invalidate its own tlb. The device can't
know and doesn't need to know whether the page-tables it used to fill
the tlb really changed.

As it looks, the problem we are seeing here is that we are sending a
large amount of these requests to the GPU device, and wait for its
completion every time. This shouldn't be a problem for ATS devices, but
the GPU here seems to fail at some point and doesn't answer to the
invalidation request anymore, causing the completion-wait loop timeouts.

Arindam's patch makes the high flush-frequency less likely, but it can
still happen, depending on how the GPU is used. So its the best to
keep ATS disabled on the device as it doesn't work correctly and we risk
running in the same problem again when we leave it enabled and just make
the trigger less likely.

	Joerg