Re: [uml-devel] Re: [PATCH 3/4] UML - Properly invoke x86_64 system calls

From: Blaisorblade
Date: Fri Nov 18 2005 - 01:37:05 EST

On Thursday 17 November 2005 21:37, linux-os (Dick Johnson) wrote:
> On Thu, 17 Nov 2005, Jeff Dike wrote:
> > This patch makes stub_segv use the stub_syscall macros. This was
> > needed anyway, but the bug that prompted this was the discovery that
> > gcc was storing stuff in RCX, which is trashed across a system
> > call.

> But the C-calling convention used by gcc for 32-bit systems
> is supposed to allow the called-function to destroy general-
> purpose registers but not index registers. In other words,
> ECX, EDX are supposed to be available and EAX is used for
> return-values. The register size isn't supposed to have
> anything to do with it (longword/quadword)!

> In 64-bit world, the same is supposed to apply as well.
> If RCX is now precious, it's a GCC bug that should be fixed.

It's not _normally_ precious, for function calling conventions, but syscalls
are different.

Read include/asm-x86_64/unistd.h and see that the _syscallX macros contain
various register clobber. I.e. they explicitly tell GCC "don't use that". We
(UML code) were missing that. Simple.
Inform me of my mistakes, so I can keep imitating Homer Simpson's "Doh!".
Paolo Giarrusso, aka Blaisorblade (Skype ID "PaoloGiarrusso", ICQ 215621894)

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