On Tue, Jun 18, 2013 at 8:45 PM, Gleb Natapov <gleb@xxxxxxxxxx> wrote:
> On Thu, Jun 13, 2013 at 05:30:03PM +0800, 李春奇 <Arthur Chunqi Li> wrote:
>> Hi Gleb,
>> I'm trying to solve these problems in the past days and meet many
>> difficulties. You want to save all the general registers in calling
>> insn_page, so registers should be saved to (save) in insn_page.
>> Because all the instructions should be generated outside and copy to
>> insn_page, and the instructions generated outside is RIP-relative, so
>> inside insn_page (save) will be wrong pointed with RIP-relative code.
>>
> They do not have to be generated outside. You can write code into
> insn_page directly. Something like this outside of any functions:
>
> asm(".align 4096\n\t"
> ".global insn_page\n\t"
> ".global insn_page_end\n\t"
> ".global test_insn\n\t"
> ".global test_insn_end\n\t"
> "insn_page:"
> "mov %%rax, outregs \n\t"
> ...
> "test_insn:\n\t"
> "in (%ds), %al\n\t"
> ". = . + 31\n\t"
> "test_insn_end:\n\t"
> "mov outregs, %%rax\n\t"
> ...
> "ret\n\t"
> ".align 4096\n\t"
> "insn_page_end:\n\t");
>
> Now you copy that into alt_insn_page, put instruction you want to test
> into test_insn offset and remap alt_insn_page into "insn_page" virtual address.
Which function can be used to remap alt_insn_page into "insn_page"
virtual address?
Arthur
>
>> I have tried to move (save) into insn_page. But when calling
>> insn_page, data in it can only be read and any instructions like "xchg
>> %%rax, 0+%[save]" may cause error, because at this time read is from
>> TLB but write will cause inconsistent.
>>
>> Another way is disabling RIP-relative code, but I failed when using
>> "-mcmodel-large -fno-pic", the binary is also using RIP-relative mode.
>> Is there any way to totally disable RIP-relative code? Besides, using
>> this feature may specified to some newer C compiler. This may not be a
>> good solution.
>>
>> If we don't set %rsp and %rbp when executing emulator code, we can
>> just use “push/pop" to save other general registers.
>>
>> If you have any better solutions, please let me know.
>>
>> Thanks,
>> Arthur
>>
>> On Thu, Jun 13, 2013 at 12:50 PM, 李春奇 <Arthur Chunqi Li>
>> <yzt356@xxxxxxxxx> wrote:
>> > On Thu, Jun 13, 2013 at 4:50 AM, Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
>> >> Il 06/06/2013 11:24, Arthur Chunqi Li ha scritto:
>> >>> Add a function trap_emulator to run an instruction in emulator.
>> >>> Set inregs first (%rax is invalid because it is used as return
>> >>> address), put instruction codec in alt_insn and call func with
>> >>> alt_insn_length. Get results in outregs.
>> >>>
>> >>> Signed-off-by: Arthur Chunqi Li <yzt356@xxxxxxxxx>
>> >>> ---
>> >>> x86/emulator.c | 81 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>> >>> 1 file changed, 81 insertions(+)
>> >>>
>> >>> diff --git a/x86/emulator.c b/x86/emulator.c
>> >>> index 96576e5..8ab9904 100644
>> >>> --- a/x86/emulator.c
>> >>> +++ b/x86/emulator.c
>> >>> @@ -11,6 +11,14 @@ 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 +693,79 @@ 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;
>> >>> +
>> >>> + // 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 general registers
>> >>> + asm volatile(
>> >>> + "push %rax\n\r"
>> >>> + "push %rbx\n\r"
>> >>> + "push %rcx\n\r"
>> >>> + "push %rdx\n\r"
>> >>> + "push %rsi\n\r"
>> >>> + "push %rdi\n\r"
>> >>> + );
>> >>
>> >> This will not work if GCC is using rsp-relative addresses to access
>> >> local variables. You need to use mov instructions to load from inregs,
>> >> and put the push/pop sequences inside the "main" asm that does the "call
>> >> *%1".
>> > Is there any way to let gcc use absolute address to access variables?
>> > I move variant "save" to the global and use "xchg %%rax, 0+%[save]"
>> > and it seems that addressing for "save" is wrong.
>> >
>> > Arthur
>> >>
>> >> Paolo
>> >>
>> >>> + // 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(
>> >>> + "call *%1\n\r"
>> >>> +
>> >>> + "mov %%rax, 0+%[outregs] \n\t"
>> >>> + "mov %%rbx, 8+%[outregs] \n\t"
>> >>> + "mov %%rcx, 16+%[outregs] \n\t"
>> >>> + "mov %%rdx, 24+%[outregs] \n\t"
>> >>> + "mov %%rsi, 32+%[outregs] \n\t"
>> >>> + "mov %%rdi, 40+%[outregs] \n\t"
>> >>> + "mov %%rsp,48+ %[outregs] \n\t"
>> >>> + "mov %%rbp, 56+%[outregs] \n\t"
>> >>> +
>> >>> + /* Save RFLAGS in outregs*/
>> >>> + "pushf \n\t"
>> >>> + "popq 72+%[outregs] \n\t"
>> >>> + : [outregs]"+m"(outregs)
>> >>> + : "r"(insn_ram),
>> >>> + "a"(mem), "b"(inregs.rbx),
>> >>> + "c"(inregs.rcx), "d"(inregs.rdx),
>> >>> + "S"(inregs.rsi), "D"(inregs.rdi)
>> >>> + : "memory", "cc"
>> >>> + );
>> >>> + // Restore general registers
>> >>> + asm volatile(
>> >>> + "pop %rax\n\r"
>> >>> + "pop %rbx\n\r"
>> >>> + "pop %rcx\n\r"
>> >>> + "pop %rdx\n\r"
>> >>> + "pop %rsi\n\r"
>> >>> + "pop %rdi\n\r"
>> >>> + );
>> >>> +}
>> >>> +
>> >>> static void advance_rip_by_3_and_note_exception(struct ex_regs *regs)
>> >>> {
>> >>> ++exceptions;
>> >>>
>> >>
>> >
>> >
>> >
>> > --
>> > Arthur Chunqi Li
>> > Department of Computer Science
>> > School of EECS
>> > Peking University
>> > Beijing, China
>>
>>
>>
>> --
>> Arthur Chunqi Li
>> Department of Computer Science
>> School of EECS
>> Peking University
>> Beijing, China
>
> --
> Gleb.
--
Arthur Chunqi Li
Department of Computer Science
School of EECS
Peking University
Beijing, China
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