Re: Kexec, DMA, and SMP

[Corey Minyard <cminyard@mvista.com>]

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Eric W. Biederman wrote:

|Corey Minyard <cminyard@mvista.com> writes:
|
|>
|>You don't understand.  You don't *want* to set aside a block of memory 
that's
|>reserved for DMA.  You want to be able to DMA directly into any user 
address.
|>Consider demand paging.  The performance would suck if you DMA into some
|>fixed region then copied to the user address.  Plus you then have another
|>resource you have to manage in the kernel.  And you still have to 
change all
|>the drivers, buffer management, etc. to add a flag that says "I'm 
going to use
|>this for DMA" to allocations.  You might as well add the quiesce 
function, it's
|>probably easier to do.  And it doesn't help if you DMA to static memory
|>addresses.
|>
|>I, too, would like a simpler solution.  I just don't think this is it.
|
|
|You have it backwards.  It is not about reserving a block of memory
|for DMA.  It is about reserving a block of memory to not do DMA in.
|Something like 4MB or so.  
|
|The idea is not to let the original kernel touch the reserved block at all.
|We just put the kernel that kexec will start in that block of memory.
|
|Eric
|
Ah, it makes much more sense now.  Thank you.  I still don't think it's 
as easy as you think, though.
Because there's no designation on most memory allocations to give you 
this information.  There's
GFP_DMA, but according to the docs that's just for x86 ISA DMA devices.  
You would
have to hunt down all the memory allocations, figure out of they are DMA 
targets or not, and add a
flag for that.  I still say it's easier to just add the function to the 
drivers.

- -Corey
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