Date:	Wed, 22 Oct 2014 11:02:54 +0800
From:	"Li, ZhenHua" <zhen-hual@...com>
To:	Bjorn Helgaas <bhelgaas@...gle.com>
CC:	Takao Indoh <indou.takao@...fujitsu.com>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linda.knippers@...com, jerry.hoemann@...com, lisa.mitchell@...com,
	alexander.duyck@...il.com, rwright@...com,
	Joerg Roedel <joro@...tes.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Tom Vaden <tom.vaden@...com>,
	David Woodhouse <dwmw2@...radead.org>,
	"open list:INTEL IOMMU (VT-d)" <iommu@...ts.linux-foundation.org>
Subject: Re: [PATCH 1/1] pci: fix dmar fault for kdump kernel

On 10/22/2014 10:47 AM, Bjorn Helgaas wrote:
> [+cc Joerg, Eric, Tom, David, iommu list]
>
> On Wed, Oct 15, 2014 at 2:14 AM, Takao Indoh <indou.takao@...fujitsu.com> wrote:
>> (2014/10/14 18:34), Li, ZhenHua wrote:
>>> I tested on the latest stable version 3.17, it works well.
>>>
>>> On 10/10/2014 03:13 PM, Li, Zhen-Hua wrote:
>>>> On a HP system with Intel vt-d supported and many PCI devices on it,
>>>> when kernel crashed and the kdump kernel boots with intel_iommu=on,
>>>> there may be some unexpected DMA requests on this adapter, which will
>>>> cause DMA Remapping faults like:
>>>>       dmar: DRHD: handling fault status reg 102
>>>>       dmar: DMAR:[DMA Read] Request device [41:00.0] fault addr fff81000
>>>>       DMAR:[fault reason 01] Present bit in root entry is clear
>>>>
>>>> This bug may happen on *any* PCI device.
>>>> Analysis for this bug:
>>>>
>>>> The present bit is set in this function:
>>>>
>>>> static struct context_entry * device_to_context_entry(
>>>>                   struct intel_iommu *iommu, u8 bus, u8 devfn)
>>>> {
>>>>       ......
>>>>                   set_root_present(root);
>>>>       ......
>>>> }
>>>>
>>>> Calling tree:
>>>>       device driver
>>>>           intel_alloc_coherent
>>>>               __intel_map_single
>>>>                   domain_context_mapping
>>>>                       domain_context_mapping_one
>>>>                           device_to_context_entry
>>>>
>>>> This means, the present bit in root entry will not be set until the device
>>>> driver is loaded.
>>>>
>>>> But in the kdump kernel, hardware devices are not aware that control has
>>>> transferred to the second kernel, and those drivers must initialize again.
>>>> Consequently there may be unexpected DMA requests from devices activity
>>>> initiated in the first kernel leading to the DMA Remapping errors in the
>>>> second kernel.
>>>>
>>>> To fix this DMAR fault, we need to reset the bus that this device on. Reset
>>>> the device itself does not work.
>
> You have not explained why the DMAR faults are a problem.  The fault
> is just an indication that the IOMMU prevented a DMA from completing.
> If the DMA is an artifact of the crashed kernel, we probably don't
> *want* it to complete, so taking a DMAR fault seems like exactly the
> right thing.
Well, I still need more time to think about other contents you mentioned 
and explained in these mails. But about the DMA fault, I think it is not 
"the iommu prevented a DMA from completing", it is the iommu could not 
help system complete the dma, so the iommu reported an error.

Also I agree with you that these DMA should not be completed.
But I still think,  these dma, programmed by the old kernel, should be 
stopped because they are some kind of illegal for the kdump kernel, no 
matter what iommu did.

>
> If the problem is that we're being flooded with messages, it's easy
> enough to just tone down the printks.
>
>>>> A patch for this bug that has been sent before:
>>>> https://lkml.org/lkml/2014/9/30/55
>>>> As in discussion, this bug may happen on *any* device, so we need to reset all
>>>> pci devices.
>>>>
>>>> There was an original version(Takao Indoh) that resets the pcie devices:
>>>> https://lkml.org/lkml/2013/5/14/9
>>
>> As far as I can remember, the original patch was nacked by
>> the following reasons:
>>
>> 1) On sparc, the IOMMU is initialized before PCI devices are enumerated,
>>     so there would still be a window where ongoing DMA could cause an
>>     IOMMU error.
>>
>> 2) Basically Bjorn is thinking device reset should be done in the
>>     1st kernel before jumping into 2nd kernel.
>
> If you're referring to this: https://lkml.org/lkml/2013/6/12/16, what
> I said was "It would be at least conceivable to reset the devices ...
> before the kexec."  That's not a requirement to do it in the first
> kernel, just an idea that I thought should be investigated.  And Eric
> has good reasons for *not* doing the reset in the first kernel, so it
> turned out not to be a very good idea.
>
> My fundamental problem with this whole reset thing is that it's a
> sledgehammer approach and it's ugly.  Using the IOMMU seems like a
> much more elegant approach.
>
> So if we are forced to accept the reset solution, I want to at least
> have a concise explanation of why we can't use the IOMMU.
>
> The changelog above is perfectly accurate, but it's really not very
> useful because it only explains the code without exploring any of the
> interesting issues.
>
> Bjorn
>
>> And Bill Sumner proposed another idea.
>> http://comments.gmane.org/gmane.linux.kernel.iommu/4828
>> I don't know the current status of this patch, but I think Jerry Hoemann
>> is working on this.
>>
>> Thanks,
>> Takao Indoh
>>
>>
>>>>
>>>> Update of this new version, comparing with Takao Indoh's version:
>>>>       Add support for legacy PCI devices.
>>>>       Use pci_try_reset_bus instead of do_downstream_device_reset in original version
>>>>
>>>> Randy Wright corrects some misunderstanding in this description.
>>>>
>>>> Signed-off-by: Li, Zhen-Hua <zhen-hual@...com>
>>>> Signed-off-by: Takao Indoh <indou.takao@...fujitsu.com>
>>>> Signed-off-by: Randy Wright <rwright@...com>
>>>> ---
>>>>    drivers/pci/pci.c | 84 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
>>>>    1 file changed, 84 insertions(+)
>>>>
>>>> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
>>>> index 2c9ac70..8cb146c 100644
>>>> --- a/drivers/pci/pci.c
>>>> +++ b/drivers/pci/pci.c
>>>> @@ -23,6 +23,7 @@
>>>>    #include <linux/device.h>
>>>>    #include <linux/pm_runtime.h>
>>>>    #include <linux/pci_hotplug.h>
>>>> +#include <linux/crash_dump.h>
>>>>    #include <asm-generic/pci-bridge.h>
>>>>    #include <asm/setup.h>
>>>>    #include "pci.h"
>>>> @@ -4423,6 +4424,89 @@ void __weak pci_fixup_cardbus(struct pci_bus *bus)
>>>>    }
>>>>    EXPORT_SYMBOL(pci_fixup_cardbus);
>>>>
>>>> +/*
>>>> + * Return true if dev is PCI root port or downstream port whose child is PCI
>>>> + * endpoint except VGA device.
>>>> + */
>>>> +static int __pci_dev_need_reset(struct pci_dev *dev)
>>>> +{
>>>> +    struct pci_bus *subordinate;
>>>> +    struct pci_dev *child;
>>>> +
>>>> +    if (dev->hdr_type != PCI_HEADER_TYPE_BRIDGE)
>>>> +        return 0;
>>>> +
>>>> +    if (pci_is_pcie(dev)) {
>>>> +        if ((pci_pcie_type(dev) != PCI_EXP_TYPE_ROOT_PORT) &&
>>>> +            (pci_pcie_type(dev) != PCI_EXP_TYPE_DOWNSTREAM))
>>>> +            return 0;
>>>> +    }
>>>> +
>>>> +    subordinate = dev->subordinate;
>>>> +    list_for_each_entry(child, &subordinate->devices, bus_list) {
>>>> +        /* Don't reset switch, bridge, VGA device */
>>>> +        if ((child->hdr_type == PCI_HEADER_TYPE_BRIDGE) ||
>>>> +            ((child->class >> 16) == PCI_BASE_CLASS_BRIDGE) ||
>>>> +            ((child->class >> 16) == PCI_BASE_CLASS_DISPLAY))
>>>> +            return 0;
>>>> +
>>>> +        if (pci_is_pcie(child)) {
>>>> +            if ((pci_pcie_type(child) == PCI_EXP_TYPE_UPSTREAM) ||
>>>> +                (pci_pcie_type(child) == PCI_EXP_TYPE_PCI_BRIDGE))
>>>> +                return 0;
>>>> +        }
>>>> +    }
>>>> +
>>>> +    return 1;
>>>> +}
>>>> +
>>>> +struct pci_dev_reset_entry {
>>>> +    struct list_head list;
>>>> +    struct pci_dev *dev;
>>>> +};
>>>> +int __init pci_reset_endpoints(void)
>>>> +{
>>>> +    struct pci_dev *dev = NULL;
>>>> +    struct pci_dev_reset_entry *pdev_entry, *tmp;
>>>> +    struct pci_bus *subordinate = NULL;
>>>> +    int has_it;
>>>> +
>>>> +    LIST_HEAD(pdev_list);
>>>> +
>>>> +    if (likely(!is_kdump_kernel()))
>>>> +        return 0;
>>>> +
>>>> +    for_each_pci_dev(dev) {
>>>> +        subordinate = dev->subordinate;
>>>> +        if (!subordinate || list_empty(&subordinate->devices))
>>>> +            continue;
>>>> +
>>>> +        has_it = 0;
>>>> +        list_for_each_entry(pdev_entry, &pdev_list, list) {
>>>> +            if (dev == pdev_entry->dev) {
>>>> +                has_it = 1;
>>>> +                break;
>>>> +            }
>>>> +        }
>>>> +        if (has_it)
>>>> +            continue;
>>>> +
>>>> +        if (__pci_dev_need_reset(dev)) {
>>>> +            pdev_entry = kmalloc(sizeof(*pdev_entry), GFP_KERNEL);
>>>> +            pdev_entry->dev = dev;
>>>> +            list_add(&pdev_entry->list, &pdev_list);
>>>> +        }
>>>> +    }
>>>> +
>>>> +    list_for_each_entry_safe(pdev_entry, tmp, &pdev_list, list) {
>>>> +        pci_try_reset_bus(pdev_entry->dev->subordinate);
>>>> +        kfree(pdev_entry);
>>>> +    }
>>>> +
>>>> +    return 0;
>>>> +}
>>>> +fs_initcall_sync(pci_reset_endpoints);
>>>> +
>>>>    static int __init pci_setup(char *str)
>>>>    {
>>>>        while (str) {
>>>>
>>>
>>>
>>>
>>
>>

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