linux-kernel - Re: [patch v2 2/4] x86, ptrace: regset extensions to support xstate

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Date:	Tue,  9 Feb 2010 17:30:31 -0800 (PST)
From:	Roland McGrath <roland@...hat.com>
To:	Suresh Siddha <suresh.b.siddha@...el.com>
Cc:	Oleg Nesterov <oleg@...hat.com>, "H. Peter Anvin" <hpa@...or.com>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>, hjl.tools@...il.com
Subject: Re: [patch v2 2/4] x86, ptrace: regset extensions to support xstate

> Add the xstate regset support which helps extend the kernel ptrace and the
> core-dump interfaces to support AVX state etc.
> 
> This regset interface is designed to support all the future state that gets
> supported using xsave/xrstor infrastructure.

Looks good modulo cosmetic nits below.

> And hence the xsave memory layout available through this regset
> interface uses SW usable bytes [464..511] to convey what state is represented
> in the memory layout.
> 
> First 8 bytes of the sw_usable_bytes[464..467] will be set to OS enabled xstate
> mask(which is same as the 64bit mask returned by the xgetbv's xCR0).

I'd like to see some macros or struct or something in some user-visible
header (ptrace-abi.h I guess?) that instruct userland where to find this
software-defined word.  The rest of the layout and meaning is specified by
the chip manuals and it's only a question of convenience if we want to help
userland out with those bits.  But the use of the reserved-for-software
area to store a bitmask (or whatever else Linux might put there in the
future) is entirely a Linux invention that needs to be a clear and explicit
part of this userland ABI format.

> +/*
> + * The xfpregs_active() routine is same as the fpregs_active() routine,

s/xfpregs_active/xstateregs_active/
s/is same/is the same/

> @@ -204,8 +209,6 @@ int xfpregs_set(struct task_struct *targ
>  	if (ret)
>  		return ret;
>  
> -	set_stopped_child_used_math(target);
> -

You didn't mention this change in a comment or log entry.
It looks like it's superfluous after init_fpu.  So this change
is right, but AFAICT it belongs in an entirely separate patch
and has nothing to do with xsave support.

> +int xstateregs_get(struct task_struct *target, const struct user_regset *regset,
> +		unsigned int pos, unsigned int count,
> +		void *kbuf, void __user *ubuf)
> +{
> +	int ret;
> +	int size = regset->n * regset->size;
[...]
> +		/*
> +		 * Copy the rest of xstate memory layout
> +		 */
> +		ret = user_regset_copyout(&pos, &count, &kbuf, &ubuf,
> +					  xsave_hdr,
> +					  offsetof(struct xsave_struct,
> +						   xsave_hdr), size);

This calculation is unnecessary (though it doesn't hurt); you can just use
-1 here.  The arch user_regset.get and user_regset.set functions are not
required to worry about bogus arguments.  Their callers are required to
pass values that don't violate the .n, .size, and .align constraints in the
user_regset.

> +int xstateregs_set(struct task_struct *target, const struct user_regset *regset,
> +		  unsigned int pos, unsigned int count,
> +		  const void *kbuf, const void __user *ubuf)
> +{
> +	int ret;
> +	int size = regset->n * regset->size;
[...]
> +	ret = user_regset_copyin(&pos, &count, &kbuf, &ubuf,
> +				 &target->thread.xstate->xsave, 0, size);

Same here.

> +	[REGSET_XSTATE] = {
> +		.core_note_type = NT_X86_XSTATE,
> +		.size = sizeof(long), .align = sizeof(long),
[...]
> +	[REGSET_XSTATE] = {
> +		.core_note_type = NT_X86_XSTATE,
> +		.size = sizeof(u32), .align = sizeof(u32),

Isn't the xsave format is really the same for 32-bit and 64-bit?
All its components are naturally 64 bits or larger, right?

If that's correct, I think it would be better to make the 32 and 64 version
of the regset uniform with .size and .align being sizeof(u64).

> +u64 xstate_fx_sw_bytes[6];
> +void update_regset_xstate_info(unsigned int size, u64 xstate_mask)
> +{
> +#ifdef CONFIG_X86_64
> +	x86_64_regsets[REGSET_XSTATE].n = size / sizeof(long);
> +#endif
> +#if defined CONFIG_X86_32 || defined CONFIG_IA32_EMULATION
> +	x86_32_regsets[REGSET_XSTATE].n = size / sizeof(u32);
> +#endif

Just change these to / sizeof(u64) accordingly.


Thanks,
Roland
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