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

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Message-ID: <583E8A9B.7070906@redhat.com>
Date:   Wed, 30 Nov 2016 16:15:23 +0800
From:   Xunlei Pang <xpang@...hat.com>
To:     Joerg Roedel <joro@...tes.org>, xlpang@...hat.com
Cc:     Myron Stowe <myron.stowe@...il.com>,
        iommu@...ts.linux-foundation.org,
        Don Brace <don.brace@...rosemi.com>,
        Dave Young <dyoung@...hat.com>, kexec@...ts.infradead.org,
        LKML <linux-kernel@...r.kernel.org>,
        Myron Stowe <myron.stowe@...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 11/29/2016 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.

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
>