Re: [PATCH 0/5] Split the emulator: decode & execute

["Laurent Vivier (Bull)" <Laurent.Vivier-6ktuUTfB/bM@public.gmane.org>]

Avi Kivity wrote:
 > Laurent Vivier wrote:
 >> Avi Kivity wrote:
...
 >>> Interesting.  I don't see how an instruction fetch can fail on
 >>> uniprocessor.  Can you give details of the failure?
 >>>
 >>> Instruction fetches can fail on SMP so a fix is certainly needed.
 >>>
 >> OK, I spoke too fast.
 >>
 >> x86_decode_insn() fails because it is not able to decode:
 >>
 >> 0xffffffff8110b7ef <__copy_user_nocache+47>:    movnti %r11,(%rdi)
 >> or
 >> 0xffffffff8110b7ef <__copy_user_nocache+47>:    0x4c    0x0f    0xc3 
    0x1f
 >>
 >> 0x4c is decoded as a REX prefix.
 >> 0x0f is decoded as a Two-byte opcode
 >> but 0xc3 is unknown in twobyte_table, so we exit because of an 
unrecognized
 >> opcode ("Cannot emulate").
 >>
 >>
 >
 > Not being able to emulate is sometimes legitimate.  In the case of
 > writing to a write-protected guest page table, we simply
 > un-write-protect it and go back to the guest (which should now execute
 > the instruction natively).
 >
 > Perhaps the logic that deals with this (the call to
 > kvm_mmu_unprotect_page_virt() in emulate_instruction()) was broken by
 > your changes.
 >

In fact this case is managed in the error cases of 
emulate_instruction(). My first patch removes this management for 
instruction decoding because I supposed it cannot generate such errors.
So what I proposed in my last email seems to be the good solution :

emulate_instruction()
...
         r = x86_decode_insn(&vcpu->emulate_ctxt, &emulate_ops);
         if (r == 0)
                 r = x86_emulate_insn(&vcpu->emulate_ctxt, &emulate_ops);
...
         if (r) {
                 if (kvm_mmu_unprotect_page_virt(vcpu, cr2))
                         return EMULATE_DONE;
                 if (!vcpu->mmio_needed) {
                         kvm_report_emulation_failure(vcpu, "mmio");
                         return EMULATE_FAIL;
                 }
                 return EMULATE_DO_MMIO;
         }
...

Regards,
Laurent

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