linux-kernel - Re: [PATCH v21 011/100] eclone (11/11): Document sys

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Date:	Wed, 9 Jun 2010 11:14:31 -0700
From:	Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com>
To:	Albert Cahalan <acahalan@...il.com>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>,
	randy.dunlap@...cle.com, linuxppc-dev@...ts.ozlabs.org,
	hpa@...or.com, roland@...hat.com, arnd@...db.de
Subject: Re: [PATCH v21 011/100] eclone (11/11): Document sys_eclone

Albert Cahalan [acahalan@...il.com] wrote:
| On Tue, Jun 1, 2010 at 9:38 PM, Sukadev Bhattiprolu
| <sukadev@...ux.vnet.ibm.com> wrote:
| > | Come on, seriously, you know it's ia64 and hppa that
| > | have issues. Maybe the nommu ports also have issues.
| > |
| > | The only portable way to specify the stack is base and offset,
| > | with flags or magic values for "share" and "kernel managed".
| >
| > Ah, ok, we have not yet ported to IA64 and I see now where the #ifdef
| > comes in.
| >
| > But are you saying that we should force x86 and other architectures to
| > specify base and offset for eclone() even though they currently specify
| > just the stack pointer to clone() ?
| 
| Even for x86, it's an easier API. Callers would be specifying
| two numbers they already have: the argument and return value
| for malloc. Currently the numbers must be added together,
| destroying information, except on hppa (must not add size)
| and ia64 (must use what I'm proposing already).

I agree its easier and would avoid #ifdefs in the applications.

Peter, Arnd, Roland - do you have any concerns with requiring all
architectures to specify the stack to eclone() as [base, offset]

To recap, currently we have 

struct clone_args {
	u64 clone_flags_high;
	/*
	 * Architectures can use child_stack for either the stack pointer or
	 * the base of of stack. If child_stack is used as the stack pointer,
	 * child_stack_size must be 0. Otherwise child_stack_size must be
	 * set to size of allocated stack.
	 */
	u64 child_stack;
	u64 child_stack_size;
	u64 parent_tid_ptr;
	u64 child_tid_ptr;
	u32 nr_pids;
	u32 reserved0;
};

sys_eclone(u32 flags_low, struct clone_args * __user cargs, int cargs_size,
		pid_t * __user pids)

Most architecutres would specify the stack pointer in ->child_stack and
ignore the ->child_stack_size.

IA64 specifies the *stack-base* in ->child_stack and the stack size in
->child_stack_size.

Albert and Randy point out that this would require #ifdefs in the
application code that intends to be portable across say IA64 and x86.

Can we instead have all architectures specify [base, size] ?

Thanks

Sukadev
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