Re: [Qemu-devel] Page protection and i386 cmpxchg8b

[Pierre d'Herbemont]

Hi Ilya!

On 23 févr. 07, at 21:32, Ilya Shar wrote:

> I'm running i386-darwin-usrer on i386 and some apps
> (Safari browser) crash because cmpxchg8b attempts to
> wrie to a qemu-allocated page which is readable but
> write-protected.  When I comment out mprotect in
> exec.c

Are you sure it does Safari does crash because of that call? I have  
the Apple Bug Reporter which complains about the fact that qemu gets  
a EXC_BAD_ACCESS, but then I get this error:
qemu: Unsupported mach syscall: -61(0xffffffc3) (=  
semaphore_signal_trap)
or
qemu: Unsupported mach syscall: -33(0xffffffdf) (=  
syscall_thread_switch)

To fix this we have to implement those syscalls.

> //        mprotect(g2h(page_addr),
> qemu_host_page_size,
> //                 (prot & PAGE_BITS) & ~PAGE_WRITE);
>
> I can proceed further.  I would guess mprotect is
> there for a reason so it doesn't seem like a good
> solution, besides the runs get substantially slower
> without mprotect.  Is there a solution to this problem
> or a way to better understand what is going on?

I think the idea behind the mprotect is to make sure that any changes  
to this pages gets monitored, and that the tb can be invalidated if  
the code was modified (self modify-ing code).

Pierre.