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SubjectRe: [PATCH] KVM: arm64: Correctly handle the mmio faulting
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Hi Keqian and Santosh,

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.

The example can be found from vfio driver in drivers/vfio/pci/vfio_pci.c:
vfio_pci_mmap: VM_PFNMAP is set for the vma
vfio_pci_mmap_fault: remap_pfn_range() is called

Thanks,
Gavin

>>
>> Now lets assume a mmio fault handing flow where guest first access
>> the MMIO region whose 2nd stage translation is not present.
>> So that results to arm64-kvm hypervisor executing guest abort handler,
>> like below:
>>
>> kvm_handle_guest_abort() -->
>> user_mem_abort()--> {
>>
>> ...
>> 0. checks the vma->flags for the VM_PFNMAP.
>> 1. Since VM_PFNMAP flag is not yet set so force_pte _is_ false;
>> 2. gfn_to_pfn_prot() -->
>> __gfn_to_pfn_memslot() -->
>> fixup_user_fault() -->
>> handle_mm_fault()-->
>> __do_fault() -->
>> vma_mmio_fault() --> // vendor's mdev fault handler
>> remap_pfn_range()--> // Here sets the VM_PFNMAP
>> flag into vma->flags.
>> 3. Now that force_pte is set to false in step-2),
>> will execute transparent_hugepage_adjust() func and
>> that lead to Oops [4].
>> }
>>
>> The proposition is to check is_iomap flag before executing the THP
>> function transparent_hugepage_adjust().
>>
>> [4] THP Oops:
>>> pc: kvm_is_transparent_hugepage+0x18/0xb0
>>> ...
>>> ...
>>> user_mem_abort+0x340/0x9b8
>>> kvm_handle_guest_abort+0x248/0x468
>>> handle_exit+0x150/0x1b0
>>> kvm_arch_vcpu_ioctl_run+0x4d4/0x778
>>> kvm_vcpu_ioctl+0x3c0/0x858
>>> ksys_ioctl+0x84/0xb8
>>> __arm64_sys_ioctl+0x28/0x38
>>
>> Tested on Huawei Kunpeng Taishan-200 arm64 server, Using VFIO-mdev device.
>> Linux tip: 583090b1
>>
>> Fixes: 6d674e28 ("KVM: arm/arm64: Properly handle faulting of device mappings")
>> Signed-off-by: Santosh Shukla <sashukla@nvidia.com>
>> ---
>> arch/arm64/kvm/mmu.c | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
>> index 3d26b47..ff15357 100644
>> --- a/arch/arm64/kvm/mmu.c
>> +++ b/arch/arm64/kvm/mmu.c
>> @@ -1947,7 +1947,7 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>> * If we are not forced to use page mapping, check if we are
>> * backed by a THP and thus use block mapping if possible.
>> */
>> - if (vma_pagesize == PAGE_SIZE && !force_pte)
>> + if (vma_pagesize == PAGE_SIZE && !force_pte && !is_iomap(flags))
>> vma_pagesize = transparent_hugepage_adjust(memslot, hva,
>> &pfn, &fault_ipa);
>> if (writable)
>>
>

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