Re: Kexec, DMA, and SMP

[Eric W. Biederman]

Corey Minyard <[e-mail]> writes:

> Eric W. Biederman wrote:
> 
> |Corey Minyard <[e-mail]> 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.

It is trivial if you don't let alloc_pages give the memory to anyone for
any purpose.

Eric