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When a REP-string is executed in 64-bit mode with an address-size prefix, ECX/EDI/ESI are used as counter and pointers. When ECX is initially zero, Intel CPUs clear the high 32-bits of RCX, and recent Intel CPUs update the high bits of the pointers in MOVS/STOS. This behavior is specific to Intel according to few experiments. As one may guess, this is an undocumented behavior. Yet, it is observable in the guest, since at least VMX traps REP-INS/OUTS even when ECX=0. Note that VMware appears to get it right. The behavior can be observed using the following code: #include <stdio.h> #define LOW_MASK (0xffffffff00000000ull) #define ALL_MASK (0xffffffffffffffffull) #define TEST(opcode) \ do { \ asm volatile(".byte 0xf2 \n\t .byte 0x67 \n\t .byte " opcode "\n\t" \ : "=S"(s), "=c"(c), "=D"(d) \ : "S"(ALL_MASK), "c"(LOW_MASK), "D"(ALL_MASK)); \ printf("opcode %s rcx=%llx rsi=%llx rdi=%llx\n", \ opcode, c, s, d); \ } while(0) void main() { unsigned long long s, d, c; iopl(3); TEST("0x6c"); TEST("0x6d"); TEST("0x6e"); TEST("0x6f"); TEST("0xa4"); TEST("0xa5"); TEST("0xa6"); TEST("0xa7"); TEST("0xaa"); TEST("0xab"); TEST("0xae"); TEST("0xaf"); } Signed-off-by: Nadav Amit <namit@cs.technion.ac.il> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> |
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