Re: Another kvm user can go away?

[Paul Goyette <paul%whooppee.com@localhost>]

On Fri, 30 Dec 2016, Simon Burge wrote:

> One thing to be careful of is 64-bit safe alignment of the sysctl
> structures.  The first structure is ok:
>
> +/* info for a single kernhist */
> +struct sysctl_history_list_entry {
> +       uint32_t        shle_nameoffset;
> +       uint32_t        shle_numentries;
> +       uint32_t        shle_nextfree;
> +       uint32_t        shle_filler;
> +};
>
> But for the second structure, I don't think the "struct timeval" is
> 64-bit alignment safe (unless something has changed in the last few
> years):
>
> +/* info for a single history event */
> +struct sysctl_history_event {
> +       struct timeval  she_tv;
> +       uint64_t        she_callnumber;
> +       uint64_t        she_values[4];
> +       uint32_t        she_cpunum;
> +       uint32_t        she_fmtoffset;
> +       uint32_t        she_funcoffset;
> +       uint32_t        she_filler;
> +};
>
> Some of the original sysctl stuff used uint32_t for the seconds part of
> a timeval, but I suspect you'd want to use a uint64_t nowadays.  You
> might end up with something like:
>
> struct sysctl_history_event {
> 	uint64_t	she_tv_sec;
> 	uint32_t	she_tv_usec;
> 	uint32_t	she_pad1;
> 	uint64_t	she_callnumber;
> 	uint64_t	she_values[4];
> 	...

Any suggestions on how to ensure proper alignment?  There doesn't seem 
to be any way to kmem_allocate() what alignment is needed...  :)  It 
seems to currently always provide 64-bit (8-byte) alignment, but the man 
page makes no guarantees.

I guess I could allocate an extra 8 bytes, and then manipulate the 
returned address... But that just "feels wrong".  :)



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