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Reply-To: Gavin Shan <gshan@redhat.com>
Subject: Re: [PATCH] KVM: arm64: Correctly handle the mmio faulting
To:     Marc Zyngier <maz@kernel.org>
Cc:     Keqian Zhu <zhukeqian1@huawei.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>
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From:   Gavin Shan <gshan@redhat.com>
Message-ID: <33eb4850-2476-01ae-e3db-5bd20c1a2c23@redhat.com>
Date:   Fri, 23 Apr 2021 11:38:38 +1000
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Hi Marc,

On 4/22/21 4:50 PM, Marc Zyngier wrote:
> On Thu, 22 Apr 2021 03:02:00 +0100,
> Gavin Shan <gshan@redhat.com> wrote:
>> 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.
>>>
>>
>> It isn't a requirement and the drivers needn't follow strictly. I checked
>> several drivers before looking into the patch and found almost all the
>> drivers have VM_PFNMAP set at mmap() time. In drivers/vfio/vfio-pci.c,
>> there is a comment as below, but it doesn't reveal too much about why
>> we can't set VM_PFNMAP at fault time.
>>
>> static int vfio_pci_mmap(void *device_data, struct vm_area_struct *vma)
>> {
>>        :
>>          /*
>>           * See remap_pfn_range(), called from vfio_pci_fault() but we can't
>>           * change vm_flags within the fault handler.  Set them now.
>>           */
>>          vma->vm_flags |= VM_IO | VM_PFNMAP | VM_DONTEXPAND | VM_DONTDUMP;
>>          vma->vm_ops = &vfio_pci_mmap_ops;
>>
>>          return 0;
>> }
>>
>> To set these flags in advance does have advantages. For example,
>> VM_DONTEXPAND prevents the vma to be merged with another
>> one. VM_DONTDUMP make this vma isn't eligible for
>> coredump. Otherwise, the address space, which is associated with the
>> vma is accessed and unnecessary page faults are triggered on
>> coredump.  VM_IO and VM_PFNMAP avoids to walk the page frames
>> associated with the vma since we don't have valid PFN in the
>> mapping.
> 
> But PCI clearly isn't the case we are dealing with here, and not
> everything is VFIO either. I can *today* create a driver that
> implements a mmap+fault handler, call mmap() on it, pass the result to
> a memslot, and get to the exact same result Santosh describes.
> 
> No PCI, no VFIO, just a random driver. We are *required* to handle
> that.
> 

hmm, ok. I was thinking it's related to VFIO mdev driver when Santosh was
talking about "mdev driver". Anyway, it's always nice to support the case :)

Thanks,
Gavin