RE: kernel memory allocations alignment

RE: kernel memory allocations alignment

[Hen, Shmulik]

Actually yes. We were warned that on IA64 architecture the system will halt
when accessing any type of variable via a pointer if the pointer does not
contain an aligned address matching that type. Until now we were using a
method of receiving a pointer to an array, casting it to a pointer of a
struct (packed with #pragma pack(1) ) ,and retrieving fields directly from
it with pointers.
It seems we cannot do that any more and were wondering what are the
alternatives.
One way we could think of is forget the packing and rearrange the fields in
the struct in descending order so they all come out aligned, but we didn't
know for sure if the first one will be aligned too.

Will that work ?


	Thanks,
	Shmulik Hen      
      Software Engineer
	Linux Advanced Networking Services
	Intel Network Communications Group
	Jerusalem, Israel

-----Original Message-----
From: Manfred [mailto:manfred@colorfullife.com]
Sent: Sunday, February 04, 2001 5:56 PM
To: Hen, Shmulik
Cc: 'LKML'
Subject: Re: kernel memory allocations alignment


"Hen, Shmulik" wrote:
> 
> When using kmalloc(size_t size), do I get a guaranty that the memory
region
> allocated is aligned according to the size specified ?
> More to the point, if I call kmalloc for type int on an IA64 architecture
is
> the pointer going to be 8 bytes aligned ?
>

Yes, kmalloc results are always 'sizeof(void*)' aligned.

Do you have stricter alignment requirements?

--
	Manfred

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Re: kernel memory allocations alignment

[Andi Kleen]

"Hen, Shmulik" <shmulik.hen@intel.com> writes:

> Actually yes. We were warned that on IA64 architecture the system will halt
> when accessing any type of variable via a pointer if the pointer does not
> contain an aligned address matching that type. Until now we were using a

That will need to be fixed with a handler anyways, the network stack requires 
unaligned accesses. If the IA64 port doesn't handle that it it's buggy and 
trivially remotely crashable.

Of course it'll always be much faster to use aligned accesses that do not
need an exception.

> method of receiving a pointer to an array, casting it to a pointer of a
> struct (packed with #pragma pack(1) ) ,and retrieving fields directly from
> it with pointers.
> It seems we cannot do that any more and were wondering what are the
> alternatives.

get_unaligned() or a memcpy to a local variable is the standard method.
get_unaligned is normally slightly faster than relying on an unalignment
exception handler.

> One way we could think of is forget the packing and rearrange the fields in
> the struct in descending order so they all come out aligned, but we didn't
> know for sure if the first one will be aligned too.
> 
> Will that work ?

Yes, it's the best solution.

-Andi
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Re: kernel memory allocations alignment

[Johannes Erdfelt]

On Sun, Feb 04, 2001, Andi Kleen <ak@suse.de> wrote:
> "Hen, Shmulik" <shmulik.hen@intel.com> writes:
> 
> > Actually yes. We were warned that on IA64 architecture the system will halt
> > when accessing any type of variable via a pointer if the pointer does not
> > contain an aligned address matching that type. Until now we were using a
> 
> That will need to be fixed with a handler anyways, the network stack requires 
> unaligned accesses. If the IA64 port doesn't handle that it it's buggy and 
> trivially remotely crashable.

That ia64 port now supports unaligned accesses in kernel mode. (via the
latest patch)

It was more a debugging aid in the beginning than inability to do.

> Of course it'll always be much faster to use aligned accesses that do not
> need an exception.

Absolutely.

JE

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kernel memory allocations alignment

[Hen, Shmulik]

Hello,

When using kmalloc(size_t size), do I get a guaranty that the memory region
allocated is aligned according to the size specified ?
More to the point, if I call kmalloc for type int on an IA64 architecture is
the pointer going to be 8 bytes aligned ?


	Shmulik Hen
	Software Engineer
	Linux Advanced Networking Services
	Network Communications Group, Israel (NCGj)
	Intel Corporation Ltd.


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Re: kernel memory allocations alignment

[Manfred]

"Hen, Shmulik" wrote:
> 
> When using kmalloc(size_t size), do I get a guaranty that the memory region
> allocated is aligned according to the size specified ?
> More to the point, if I call kmalloc for type int on an IA64 architecture is
> the pointer going to be 8 bytes aligned ?
>

Yes, kmalloc results are always 'sizeof(void*)' aligned.

Do you have stricter alignment requirements?

--
	Manfred
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