[gem5-users] Re: Privileged Instruction in Full System Mode

[Jason Lowe-Power via gem5-users]

Hi David,

That is very strange. I would also expect gem5 to produce a protection
fault on that code. I would try a couple of things:
1. Try running interactively in gem5 (e.g., with m5term) and seeing what
happens
2. Try using gdb interactive *in gem5*. In other words, run gdb inside the
simulator
3. Try using the KVM CPU which will allow you to boot linux and run your
program in seconds instead of hours :).

gem5 should not be missing any protection checks. It is possible that an
instruction was mis-classified as being a user-mode instruction and not
privileged, but that's unlikely.

Cheers,
Jason

On Thu, Aug 27, 2020 at 10:56 AM David Klopp via gem5-users <
gem5-users@gem5.org> wrote:

> Hello,
>
> I have a questions regarding executing privileged instructions in gem5. I
> tested the following program on the AtomicSimple and O3 CPU type (arch x86)
> inside gem5 (version 20.0.0.2) running a full system Linux with kernel
> version 5.4.55:
>
> static inline void invlpg(unsigned long addr) {
>     asm volatile("invlpg (%0)" ::"r" (addr) : "memory");
> }
>
> int main() {
>     int mem;
>     invlpg((unsigned long) &mem);
> }
>
> The program was executed without an error. I would have expected, that
> some kind of error occurs, because invlpg is a privileged instruction, so I
> should not be able to execute it in user mode. To verify that this
> behaviour is indeed special to gem5 and not related to my kernel, disk
> image or program, I booted the same Linux system with the same kernel and
> disk image in qemu and executed the same binary. Executing the program in
> qemu resulted in a general protection fault.
> Is gem5 missing some kind of check which prevents executing privileged
> instruction in user mode, or is this somehow expected behaviour and I
> missed something ?
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