linux-kernel - Re: [PATCH] iommu/vt-d: Flush old iotlb for kdump when the device gets context mapped

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Date:   Wed, 30 Nov 2016 18:23:34 +0800
From:   Baoquan He <bhe@...hat.com>
To:     xlpang@...hat.com
Cc:     Joerg Roedel <joro@...tes.org>,
        Don Brace <don.brace@...rosemi.com>,
        Myron Stowe <myron.stowe@...hat.com>,
        kexec@...ts.infradead.org, LKML <linux-kernel@...r.kernel.org>,
        iommu@...ts.linux-foundation.org,
        Myron Stowe <myron.stowe@...il.com>,
        Dave Young <dyoung@...hat.com>,
        David Woodhouse <dwmw2@...radead.org>
Subject: Re: [PATCH] iommu/vt-d: Flush old iotlb for kdump when the device
 gets context mapped

On 11/30/16 at 05:53pm, Baoquan He wrote:
> On 11/30/16 at 05:03pm, Baoquan He wrote:
> > On 11/30/16 at 04:15pm, Xunlei Pang wrote:
> > > On 11/29/2016 at 10:35 PM, Joerg Roedel wrote:
> > > > On Thu, Nov 17, 2016 at 10:47:28AM +0800, Xunlei Pang wrote:
> > > >> As per the comment, the code here only needs to flush context caches
> > > >> for the special domain 0 which is used to tag the
> > > >> non-present/erroneous caches, seems we should flush the old domain id
> > > >> of present entries for kdump according to the analysis, other than the
> > > >> new-allocated domain id. Let me ponder more on this.
> > > > Flushing the context entry only is fine. The old domain-id will not be
> > > > re-used anyway, so there is no point in reading it out of the context
> > > > table and flush it.
> > > 
> > > Do you mean to flush the context entry using the new-allocated domain id?
> > > 
> > > Yes, old domain-id will not be re-used as they were reserved when copy, but
> > > may still be cached by in-flight DMA access.
> > 
> > Joerg is saying you have flushed context entry which is the ingress,
> > new DMA can't get an entrance to hit the iotlb accordingly. Since you
> > have bolted the ingress gate. I guess
> 

OK, talked with Xunlei. The old cache could be entry with present bit
set.

> And please code comment at the bottom of iommu_init_domains(), you can
> see domain 0 is a special domain id.
> 
> 
> ~~~~~~~~~~~~~~~~~~~~~~~~~
>         /*
>          * If Caching mode is set, then invalid translations are tagged
>          * with domain-id 0, hence we need to pre-allocate it. We also
>          * use domain-id 0 as a marker for non-allocated domain-id, so
>          * make sure it is not used for a real domain.
>          */
>         set_bit(0, iommu->domain_ids);
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 
> And in vt-d spec, at the end of section 6.2.2 and the following
> sections, you can see domain 0 is used to tag the cached entry.
> 
> I guess that's why it works with only domain 0 specified. The simple
> thing to verify that is you specify another did, E.g 100 for your
> flushing, see if it still works.
> 
> 
> So, if it's just as above, v1 should be good enough.
> 
> Besides, you should use translation_pre_enabled(). If 1st kernel add
> intel_iommu=off, no need to do this.
> 
> Thanks
> Baoquan
> > 
> > > 
> > > Here is what the things seem to be from my understanding, and why I want to
> > > flush using the old domain id:
> > > 1) In kdump mode, old tables are copied, and all the iommu caches are flushed.
> > > 2) There comes some in-flight DMA before the device's new context is mapped,
> > >     so translation caches(context, iotlb, etc) are created tagging old domain-id
> > >     in the iommu hardware.
> > > 3) At the driver probe stage, the device is reset , and no in-flight DMA will exist.
> > >     Here I assumed that the device reset won't flush the old caches in the iommu
> > >     hardware related to this device. I haven't found any relevant specification, please
> > >     correct me if I am wrong.
> > > 4) Then new context is setup, and new DMA is initiated, hit old cache that was
> > >     created in 2) as currently there's no such flush action, so DMAR fault happens.
> > > 
> > > I already posted v2 to flush context/iotlb using the old domain-id:
> > > https://lkml.org/lkml/2016/11/18/514
> > > 
> > > Regards,
> > > Xunlei
> > > 
> > > >
> > > > Also, please add a Fixes-tag when you re-post this patch.
> > > >
> > > >
> > > > 	Joerg
> > > >
> > > 
> > > 
> > > _______________________________________________
> > > kexec mailing list
> > > kexec@...ts.infradead.org
> > > http://lists.infradead.org/mailman/listinfo/kexec

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