linux-kernel - Re: [PATCH v1 1/8] tools/nolibc: add support for powerpc

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Date:   Wed, 26 Jul 2023 04:23:19 +0800
From:   Zhangjin Wu <falcon@...ylab.org>
To:     ammarfaizi2@...weeb.org
Cc:     arnd@...db.de, falcon@...ylab.org, linux-kernel@...r.kernel.org,
        linux-kselftest@...r.kernel.org, svens@...ux.ibm.com,
        thomas@...ch.de, w@....eu
Subject: Re: [PATCH v1 1/8] tools/nolibc: add support for powerpc

> On Wed, Jul 26, 2023 at 01:04:26AM +0800, Zhangjin Wu wrote:
> > My old 'reply' is not rigorous, since the syscall6() uses stack to pass
> > the 6th argument, so, our new syscall.h didn't support it currently,
> > the syscalls I have tested about "=r" instead of "=a" were only syscall1-5().
> 
> Yeah, it won't fit with the new design.
> 
> i386 runs out of GPRs very quickly. Given that, it had a hard time
> implementing syscall6() properly in nolibc. The calling convention
> itself actually doesn't require stack for executing 'int $0x80'.
> 
> The reason of why it uses stack is because the %ebp register cannot be
> listed in the clobber list nor in the constraint if -fomit-frame-pointer
> is not activated. Thus, we have to carefully preserve the value on the
> stack before using %ebp as the 6-th argument to the syscall. It's a hack
> to make it work on i386.
> 
> > Ok, so, with the new syscalls.h proposed, we'd better keep i386
> > syscall6() as-is.
> > 
> > For the left syscall1-5(), is there any risk when use '=r' instead of 'r'?
> 
> Using "=r" instead of "r" doesn't make sense.
> 
> Did you mean "=r" instead of "=a"?
>

Yeah, sorry.

> If that's what you mean:
> 
> So far I don't see the risk of using "=r" instead of "=a" as long as the
> variable is properly marked as 'register' + asm("eax").
>

Thanks.
Zhangjin

> -- 
> Ammar Faizi