Hi Marc, Thanks for the review comment.
Hi Santosh,
Thanks for this.
On 2020-10-21 17:16, 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.
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].
}
Hmmm. Nice. Any chance you could provide us with an actual reproducer?
I tried to create the reproducer scenario with vfio-pci driver
using
nvidia GPU in PT mode, As because vfio-pci driver now supports
vma faulting (/vfio_pci_mmap_fault) so could create a crude
reproducer
situation with that.
To create the repro - I did an ugly hack into arm64/kvm/mmu.c.
The hack is to make sure that stage2 mapping are not created
at the time of vm_init by unsetting VM_PFNMAP flag. This
`unsetting` flag
needed because vfio-pci's mmap func(/vfio_pci_mmap) by-default
sets the VM_PFNMAP flag for the MMIO region but I want
the remap_pfn_range() func to set the _PFNMAP flag via vfio's
fault
handler func vfio_pci_mmap_fault().
So with above, when guest access the MMIO region, this will
trigger the mmio fault path at arm64-kvm hypervisor layer like
below:
user_mem_abort() {->...
--> Check the VM_PFNMAP flag, since not set so marks
force_pte=false
....
__gfn_to_pfn_memslot()-->
...
handle_mm_fault()-->
do_fault()-->
vfio_pci_mmio_fault()-->
remap_pfn_range()--> Now will set the VM_PFNMAP flag.
}
I have also set the force_pte=true, just to avoid the THP Oops
which was
mentioned in the previous thread.
hackish change to reproduce scenario:
--->
diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index 19aacc7d64de..9ef70dc624cf 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -836,9 +836,9 @@ static int user_mem_abort(struct kvm_vcpu
*vcpu, phys_addr_t fault_ipa,
}
if (is_error_noslot_pfn(pfn))
return -EFAULT;
-
if (kvm_is_device_pfn(pfn)) {
device = true;
+ force_pte = true;
} else if (logging_active && !write_fault) {
/*
* Only actually map the page as writable if this
was a write
@@ -1317,6 +1317,11 @@ int kvm_arch_prepare_memory_region(struct
kvm *kvm,
vm_start = max(hva, vma->vm_start);
vm_end = min(reg_end, vma->vm_end);
+ /* Hack to make sure stage2 mapping not present,
thus trigger
+ * user_mem_abort for stage2 mapping */
+ if (vma->vm_flags & VM_PFNMAP) {
+ vma->vm_flags = vma->vm_flags &
(~VM_PFNMAP);
+ }
if (vma->vm_flags & VM_PFNMAP) {
gpa_t gpa = mem->guest_phys_addr +
(vm_start -
mem->userspace_addr);
Yes.
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)
Why don't you directly set force_pte to true at the point where we
update
the flags? It certainly would be a bit more readable:
diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index 3d26b47a1343..7a4ad984d54e 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -1920,6 +1920,7 @@ static int user_mem_abort(struct kvm_vcpu *vcpu,
phys_addr_t fault_ipa,
if (kvm_is_device_pfn(pfn)) {
mem_type = PAGE_S2_DEVICE;
flags |= KVM_S2PTE_FLAG_IS_IOMAP;
+ force_pte = true;
} else if (logging_active) {
/*
* Faults on pages in a memslot with logging enabled
and almost directly applies to what we have queued for 5.10.
Right. I believe - Above code is sightly changed at linux-next
commit: 9695c4ff
Modified one looks like below:
diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index 19aacc7..d4cd253 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -839,6 +839,7 @@ static int user_mem_abort(struct kvm_vcpu
*vcpu, phys_addr_t fault_ipa,
if (kvm_is_device_pfn(pfn)) {
device = true;
+ force_pte = true;
} else if (logging_active && !write_fault) {
/*
* Only actually map the page as writable if this
was a write
pl. let me know if above is okay and will send out v2.
Thanks.
Santosh
Thanks,
M.
--
Jazz is not dead. It just smells funny...
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