Messages in this thread | | | Subject | Re: [PATCH] KVM: arm64: Correctly handle the mmio faulting | From | Gavin Shan <> | Date | Wed, 21 Apr 2021 16:20:12 +1000 |
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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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