[Xen-devel] Re: Possible regression in "x86-64: reduce range spanned by 1:1 mapping and frame table indexes"

["Jan Beulich" <JBeulich@xxxxxxxxxx>]

>(XEN) ----[ Xen-3.5-unstable  x86_64  debug=y  Not tainted ]----
>(XEN) CPU:    0
>(XEN) RIP:    e008:[<ffff82c4801c0a72>] sh_remove_shadows+0x149/0x86a
>(XEN) RFLAGS: 0000000000010286   CONTEXT: hypervisor
>(XEN) rax: ffff8301140fa000   rbx: ffff82f601e10000   rcx: 0000000000000000
>(XEN) rdx: ffff82c4801fb9e0   rsi: 00000000000f0800   rdi: ffff8301140fae1c
>(XEN) rbp: ffff82c4802dfb68   rsp: ffff82c4802dfb38   r8:  00000000000f0800
>(XEN) r9:  ffff8301186a8000   r10: ffff8300ddc08000   r11: 0000000000000000
>(XEN) r12: ffff8300ddc08000   r13: 00000000000f0800   r14: ffff82c4802dff28
>(XEN) r15: 0000000000000000   cr0: 0000000080050033   cr4: 00000000000026f0
>...
>(XEN) Xen call trace:
>(XEN)    [<ffff82c4801c0a72>] sh_remove_shadows+0x149/0x86a
>(XEN)    [<ffff82c4801cbfff>] sh_page_fault__guest_2+0x184d/0x1bf3
>(XEN)    [<ffff82c4801b2c1d>] vmx_vmexit_handler+0x717/0x1a68
>(XEN)    
>(XEN) Pagetable walk from ffff82f601e1000f:
>(XEN)  L4[0x105] = 00000000decea027 5555555555555555
>(XEN)  L3[0x1d8] = 000000011bffb063 5555555555555555
>(XEN)  L2[0x00f] = 0000000000000000 ffffffffffffffff 
>(XEN) 
>(XEN) ****************************************
>(XEN) Panic on CPU 0:
>(XEN) FATAL PAGE FAULT
>(XEN) [error_code=0000]
>(XEN) Faulting linear address: ffff82f601e1000f
>(XEN) ****************************************

While I can't determine the exact source location corresponding to the
crash (without the disassembly of the function), the page table walk
suggests this is a read from to the M2P table, which imposes a couple
of questions: How can this be a non-quad-word aligned access? Is the
access, if it makes sense, guarded by an mfn_valid() check? Is the
memory address corresponding to the M2P slot (mfn 0x3c20001) in a
physical memory hole?

Jan


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