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

From: Baoquan He <bhe-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
To: xlpang-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org
Cc: Don Brace <don.brace-dzo6w/eZyo2tG0bUXCXiUA@public.gmane.org>,
	Dave Young <dyoung-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>,
	kexec-IAPFreCvJWM7uuMidbF8XUB+6BGkLq7r@public.gmane.org,
	LKML <linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>,
	iommu-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org,
	Myron Stowe <myron.stowe-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>,
	David Woodhouse <dwmw2-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>
Subject: Re: [PATCH] iommu/vt-d: Flush old iotlb for kdump when the device gets context mapped
Date: Wed, 30 Nov 2016 17:03:27 +0800	[thread overview]
Message-ID: <20161130090327.GA4192@x1> (raw)
In-Reply-To: <583E8A9B.7070906-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>

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

> 
> 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
> >
> 
> 
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