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Subject: Re: [PATCH] KVM: arm64: Correctly handle the mmio faulting
To:     Santosh Shukla <santosh.shukla1982@gmail.com>,
        "Tarun Gupta (SW-GPU)" <targupta@nvidia.com>
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CC:     Marc Zyngier <maz@kernel.org>, Gavin Shan <gshan@redhat.com>,
        <kvm@vger.kernel.org>, <kvmarm@lists.cs.columbia.edu>,
        <linux-kernel@vger.kernel.org>, <cjia@nvidia.com>,
        <linux-arm-kernel@lists.infradead.org>,
        "Wanghaibin (D)" <wanghaibin.wang@huawei.com>
From:   Keqian Zhu <zhukeqian1@huawei.com>
Message-ID: <c8b479d0-0766-ecb7-49fb-d94b715b0922@huawei.com>
Date:   Fri, 23 Apr 2021 09:06:20 +0800
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On 2021/4/22 16:00, Santosh Shukla wrote:
> On Thu, Apr 22, 2021 at 1:07 PM Tarun Gupta (SW-GPU)
> <targupta@nvidia.com> wrote:
>>
>>
>>
>> On 4/22/2021 12:20 PM, Marc Zyngier wrote:
>>> External email: Use caution opening links or attachments
>>>
>>>
>>> On Thu, 22 Apr 2021 03:02:00 +0100,
>>> Gavin Shan <gshan@redhat.com> wrote:
>>>>
>>>> Hi Marc,
>>>>
>>>> On 4/21/21 9:59 PM, Marc Zyngier wrote:
>>>>> On Wed, 21 Apr 2021 07:17:44 +0100,
>>>>> Keqian Zhu <zhukeqian1@huawei.com> wrote:
>>>>>> On 2021/4/21 14:20, Gavin Shan wrote:
>>>>>>> On 4/21/21 12:59 PM, Keqian Zhu wrote:
>>>>>>>> On 2020/10/22 0:16, Santosh Shukla wrote:
>>>>>>>>> The Commit:6d674e28 introduces a notion to detect and handle the
>>>>>>>>> device mapping. The commit checks for the VM_PFNMAP flag is set
>>>>>>>>> in vma->flags and if set then marks force_pte to true such that
>>>>>>>>> if force_pte is true then ignore the THP function check
>>>>>>>>> (/transparent_hugepage_adjust()).
>>>>>>>>>
>>>>>>>>> There could be an issue with the VM_PFNMAP flag setting and checking.
>>>>>>>>> For example consider a case where the mdev vendor driver register's
>>>>>>>>> the vma_fault handler named vma_mmio_fault(), which maps the
>>>>>>>>> host MMIO region in-turn calls remap_pfn_range() and maps
>>>>>>>>> the MMIO's vma space. Where, remap_pfn_range implicitly sets
>>>>>>>>> the VM_PFNMAP flag into vma->flags.
>>>>>>>> Could you give the name of the mdev vendor driver that triggers this issue?
>>>>>>>> I failed to find one according to your description. Thanks.
>>>>>>>>
>>>>>>>
>>>>>>> I think it would be fixed in driver side to set VM_PFNMAP in
>>>>>>> its mmap() callback (call_mmap()), like vfio PCI driver does.
>>>>>>> It means it won't be delayed until page fault is issued and
>>>>>>> remap_pfn_range() is called. It's determined from the beginning
>>>>>>> that the vma associated the mdev vendor driver is serving as
>>>>>>> PFN remapping purpose. So the vma should be populated completely,
>>>>>>> including the VM_PFNMAP flag before it becomes visible to user
>>>>>>> space.
>>>>>
>>>>> Why should that be a requirement? Lazy populating of the VMA should be
>>>>> perfectly acceptable if the fault can only happen on the CPU side.
>>>>>
> 
> Right.
> Hi keqian,
> You can refer to case
> http://lkml.iu.edu/hypermail/linux/kernel/2010.3/00952.html
Hi Santosh,

Yeah, thanks for that.

BRs,
Keqian