On Sun, Jun 9, 2013 at 7:07 PM, Gleb Natapov <gleb@xxxxxxxxxx> wrote:
> On Fri, Jun 07, 2013 at 10:31:38AM +0800, Arthur Chunqi Li wrote:
>> Add a function trap_emulator to run an instruction in emulator.
>> Set inregs first (%rax, %rsp, %rbp, %rflags have special usage and
>> cannot set in inregs), put instruction codec in alt_insn and call
>> func with alt_insn_length. Get results in outregs.
>>
> Why %rax, %rsp, %rbp, %rflags cannot be set in inregs?
>
> %rax because trapping instruction uses it? Use one that does not use
> register at all: MOV r/m32, imm32
I don't know why set %rax before call alt_insn_page can cause error. I
use "xchg %%rax, 0+%[save]" before "call *%1" and the %rcx is not set
correctly.
>
> %rsp and %rbp because of ret on instruction page? Use the same trick
> realmode.c test uses: have the code that sets/saves registers in
> insn_page/alt_insn_page itself and copy the instruction you want to test
> into the page itself instead of doing call.
I don't know how instructions between calling insn_page and
alt_insn_page are executed (function install_page and some other
instructions before call *%1". If these insns are executed after
insn_page is called, changes before the trapping instruction may
affect the executing of these instructions.
>
> Not sure what is so special about %rflags.
>
>> Signed-off-by: Arthur Chunqi Li <yzt356@xxxxxxxxx>
>> ---
>> x86/emulator.c | 67 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>> 1 file changed, 67 insertions(+)
>> mode change 100644 => 100755 x86/emulator.c
>>
>> diff --git a/x86/emulator.c b/x86/emulator.c
>> old mode 100644
>> new mode 100755
>> index 96576e5..770e8f7
>> --- a/x86/emulator.c
>> +++ b/x86/emulator.c
>> @@ -11,6 +11,13 @@ int fails, tests;
>>
>> static int exceptions;
>>
>> +struct regs {
>> + u64 rax, rbx, rcx, rdx;
>> + u64 rsi, rdi, rsp, rbp;
>> + u64 rip, rflags;
>> +};
>> +static struct regs inregs, outregs;
>> +
>> void report(const char *name, int result)
>> {
>> ++tests;
>> @@ -685,6 +692,66 @@ static void test_shld_shrd(u32 *mem)
>> report("shrd (cl)", *mem == ((0x12345678 >> 3) | (5u << 29)));
>> }
>>
>> +static void trap_emulator(uint64_t *mem, uint8_t *insn_page,
>> + uint8_t *alt_insn_page, void *insn_ram,
>> + uint8_t *alt_insn, int alt_insn_length)
>> +{
>> + ulong *cr3 = (ulong *)read_cr3();
>> + int i;
>> + static struct regs save;
>> +
>> + // Pad with RET instructions
>> + memset(insn_page, 0xc3, 4096);
>> + memset(alt_insn_page, 0xc3, 4096);
>> +
>> + // Place a trapping instruction in the page to trigger a VMEXIT
>> + insn_page[0] = 0x89; // mov %eax, (%rax)
>> + insn_page[1] = 0x00;
>> + insn_page[2] = 0x90; // nop
>> + insn_page[3] = 0xc3; // ret
>> +
>> + // Place the instruction we want the hypervisor to see in the alternate page
>> + for (i=0; i<alt_insn_length; i++)
>> + alt_insn_page[i] = alt_insn[i];
>> + save = inregs;
>> +
>> + // Load the code TLB with insn_page, but point the page tables at
>> + // alt_insn_page (and keep the data TLB clear, for AMD decode assist).
>> + // This will make the CPU trap on the insn_page instruction but the
>> + // hypervisor will see alt_insn_page.
>> + install_page(cr3, virt_to_phys(insn_page), insn_ram);
>> + invlpg(insn_ram);
>> + // Load code TLB
>> + asm volatile("call *%0" : : "r"(insn_ram + 3));
>> + install_page(cr3, virt_to_phys(alt_insn_page), insn_ram);
>> + // Trap, let hypervisor emulate at alt_insn_page
>> + asm volatile(
>> + "xchg %%rbx, 8+%[save] \n\t"
>> + "xchg %%rcx, 16+%[save] \n\t"
>> + "xchg %%rdx, 24+%[save] \n\t"
>> + "xchg %%rsi, 32+%[save] \n\t"
>> + "xchg %%rdi, 40+%[save] \n\t"
>> +
>> + "call *%1\n\t"
>> +
>> + "mov %%rax, 0+%[save] \n\t"
>> + "xchg %%rbx, 8+%[save] \n\t"
>> + "xchg %%rcx, 16+%[save] \n\t"
>> + "xchg %%rdx, 24+%[save] \n\t"
>> + "xchg %%rsi, 32+%[save] \n\t"
>> + "xchg %%rdi, 40+%[save] \n\t"
>> + "mov %%rsp, 48+%[save] \n\t"
>> + "mov %%rbp, 56+%[save] \n\t"
>> + /* Save RFLAGS in outregs*/
>> + "pushf \n\t"
>> + "popq 72+%[save] \n\t"
>> + : [save]"+m"(save)
>> + : "r"(insn_ram), "a"(mem)
>> + : "memory", "cc"
>> + );
>> + outregs = save;
>> +}
>> +
>> static void advance_rip_by_3_and_note_exception(struct ex_regs *regs)
>> {
>> ++exceptions;
>> --
>> 1.7.9.5
>
> --
> Gleb.
--
Arthur Chunqi Li
Department of Computer Science
School of EECS
Peking University
Beijing, China
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