Messages in this thread | | | Subject | Re: [PATCH] KVM: arm64: Correctly handle the mmio faulting | From | Gavin Shan <> | 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
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