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@xxxxxxxxxx>
---
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)
