Re: [PATCH v9 6/6] KVM: x86/mmu: Handle non-refcounted pages

[Dmitry Osipenko <dmitry.osipenko@collabora.com>]

On 9/18/23 12:58, Dmitry Osipenko wrote:
> On 9/11/23 05:16, David Stevens wrote:
>> From: David Stevens <stevensd@chromium.org>
>>
>> Handle non-refcounted pages in __kvm_faultin_pfn. This allows the host
>> to map memory into the guest that is backed by non-refcounted struct
>> pages - for example, the tail pages of higher order non-compound pages
>> allocated by the amdgpu driver via ttm_pool_alloc_page.
>>
>> The bulk of this change is tracking the is_refcounted_page flag so that
>> non-refcounted pages don't trigger page_count() == 0 warnings. This is
>> done by storing the flag in an unused bit in the sptes. There are no
>> bits available in PAE SPTEs, so non-refcounted pages can only be handled
>> on TDP and x86-64.
>>
>> Signed-off-by: David Stevens <stevensd@chromium.org>
>> ---
>>  arch/x86/kvm/mmu/mmu.c          | 52 +++++++++++++++++++++++----------
>>  arch/x86/kvm/mmu/mmu_internal.h |  1 +
>>  arch/x86/kvm/mmu/paging_tmpl.h  |  8 +++--
>>  arch/x86/kvm/mmu/spte.c         |  4 ++-
>>  arch/x86/kvm/mmu/spte.h         | 12 +++++++-
>>  arch/x86/kvm/mmu/tdp_mmu.c      | 22 ++++++++------
>>  include/linux/kvm_host.h        |  3 ++
>>  virt/kvm/kvm_main.c             |  6 ++--
>>  8 files changed, 76 insertions(+), 32 deletions(-)
> 
> Could you please tell which kernel tree you used for the base of this
> series? This patch #6 doesn't apply cleanly to stable/mainline/next/kvm
> 
> error: sha1 information is lacking or useless (arch/x86/kvm/mmu/mmu.c).
> error: could not build fake ancestor

I applied the patch manually to v6.5.2 and tested Venus using Intel TGL iGPU, the intel driver is crashing:

   BUG: kernel NULL pointer dereference, address: 0000000000000058
   #PF: supervisor read access in kernel mode
   #PF: error_code(0x0000) - not-present page
   PGD 0 P4D 0 
   Oops: 0000 [#1] PREEMPT SMP
   CPU: 1 PID: 5926 Comm: qemu-system-x86 Not tainted 6.5.2+ #114
   Hardware name: LENOVO 20VE/LNVNB161216, BIOS F8CN43WW(V2.06) 08/12/2021
   RIP: 0010:gen8_ppgtt_insert+0x50b/0x8f0
   Code: 00 00 f7 c2 00 00 20 00 74 15 f7 c3 ff ff 1f 00 75 0d 41 81 fc ff ff 1f 00 0f 87 0e 02 00 00 48 8b 74 24 08 44 89 c0 45 85 ed <48> 8b 4e 58 48 8b 04 c1 0f 85 0b 02 00 00 81 e2 00 00 01 00 0f 84
   RSP: 0018:ffffafc085afb820 EFLAGS: 00010246
   RAX: 0000000000000000 RBX: 00000000e9604000 RCX: 000000000000001b
   RDX: 0000000000211000 RSI: 0000000000000000 RDI: ffff9513d44c1000
   RBP: ffff951106f8dfc0 R08: 0000000000000000 R09: 0000000000000003
   R10: 0000000000000fff R11: 00000000e9800000 R12: 00000000001fc000
   R13: 0000000000000000 R14: 0000000000001000 R15: 0000ffff00000000
   FS:  00007f2a5bcced80(0000) GS:ffff951a87a40000(0000) knlGS:0000000000000000
   CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
   CR2: 0000000000000058 CR3: 0000000116f16006 CR4: 0000000000772ee0
   PKRU: 55555554
   Call Trace:
    <TASK>
    ? __die+0x1f/0x60
    ? page_fault_oops+0x14d/0x420
    ? exc_page_fault+0x3d7/0x880
    ? lock_acquire+0xc9/0x290
    ? asm_exc_page_fault+0x22/0x30
    ? gen8_ppgtt_insert+0x50b/0x8f0
    ppgtt_bind_vma+0x4f/0x60
    fence_work+0x1b/0x70
    fence_notify+0x8f/0x130
    __i915_sw_fence_complete+0x58/0x230
    i915_vma_pin_ww+0x513/0xa80
    eb_validate_vmas+0x17e/0x9e0
    ? eb_pin_engine+0x2bb/0x340
    i915_gem_do_execbuffer+0xc85/0x2bf0
    ? __lock_acquire+0x3b6/0x21c0
    i915_gem_execbuffer2_ioctl+0xee/0x240
    ? i915_gem_do_execbuffer+0x2bf0/0x2bf0
    drm_ioctl_kernel+0x9d/0x140
    drm_ioctl+0x1dd/0x410
    ? i915_gem_do_execbuffer+0x2bf0/0x2bf0
    ? __fget_files+0xc5/0x170
    __x64_sys_ioctl+0x8c/0xc0
    do_syscall_64+0x34/0x80
    entry_SYSCALL_64_after_hwframe+0x46/0xb0
   RIP: 0033:0x7f2a60b0c9df


$ ./scripts/faddr2line ./vmlinux gen8_ppgtt_insert+0x50b/0x8f0
gen8_ppgtt_insert+0x50b/0x8f0:
i915_pt_entry at drivers/gpu/drm/i915/gt/intel_gtt.h:557
(inlined by) gen8_ppgtt_insert_huge at drivers/gpu/drm/i915/gt/gen8_ppgtt.c:641
(inlined by) gen8_ppgtt_insert at drivers/gpu/drm/i915/gt/gen8_ppgtt.c:743

It's likely should be the i915 driver issue that is crashes with the NULL deref, but the origin of the bug should be the kvm page fault handling. 

David, could you please tell what tests you've run and post a link to yours kernel tree? Maybe I made obscure mistake while applied the patch manually.

-- 
Best regards,
Dmitry