Valgrind is changing the values returned by the CPUID opcode instruction. Simply put, how can I make Valgrind respect the actual CPUID instruction?
For reference, this was discovered when running into strange errors when detecting aes-ni support on an old computer which I know does not have the aes-ni instruction set. This behavior, however, is clearly changing multiple values.
This behavior can be observed with valgrind-3.10.1, using the following C code:
#include <stdio.h>
int main() {
unsigned eax, ebx, ecx, edx;
eax = 1;
__asm__ volatile("cpuid"
: "=a" (eax), "=b" (ebx), "=c" (ecx), "=d" (edx)
: "0" (eax), "2" (ecx)
);
if(ecx & (1<<25)) {
printf("aes-ni enabled (ecx=%08x)n", ecx);
} else {
printf("no aes-ni support (ecx=%08x)\n", ecx);
}
return 1;
}
Which compiles and runs as such:
$ gcc -o test test.c
$ ./test
no aes-ni support (ecx=0098e3fd)
$ valgrind ./test
==25361== Memcheck, a memory error detector
==25361== Copyright (C) 2002-2013, and GNU GPL'd, by Julian Seward et al.
==25361== Using Valgrind-3.10.1 and LibVEX; rerun with -h for copyright info
==25361== Command: ./test
==25361==
aes-ni enabled (ecx=0298e3ff)
==25361==
==25361== HEAP SUMMARY:
==25361== in use at exit: 0 bytes in 0 blocks
==25361== total heap usage: 0 allocs, 0 frees, 0 bytes allocated
==25361==
==25361== All heap blocks were freed -- no leaks are possible
==25361==
==25361== For counts of detected and suppressed errors, rerun with: -v
==25361== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)
Note that the same binary returns 0098e3fd normally, but 0298e3ff under valgrind, which is wrong!
After a couple days with no answers, it would appear Valgrind is incapable of allowing a correct CPUID response.
Because Valgrind is, essentially, running inside a virtual environment, it will respond CPUID information about the virtual processor it's aware of, and not the system's processor.
Thanks to a comment by @Joe the following link shows a conversation about this dating back to 2014: https://sourceforge.net/p/valgrind/mailman/message/31960632/
In short, it would be nice for Valgrind to have an option to set CPUID flags as a runtime flag (as was suggested in the linked thread), but to date (February 2018) no such flag exists.