Re: [PATCH] x86: Voluntary leave_mm before entering ACPI C3

[Venki Pallipadi <venkatesh.pallipadi@intel.com>]

On Wed, Dec 19, 2007 at 11:48:14AM -0800, H. Peter Anvin wrote:
> Ingo Molnar wrote:
> >
> >i dont think it's required for C3 to even turn off any portion of the 
> >CPU - if an interrupt arrives after the C3 sequence is initiated but 
> >just before dirty cachelines have been flushed then the CPU can just 
> >return without touching anything (such as the TLB) - right? So i dont 
> >think there's any implicit guarantee of TLB flushing (nor should there 
> >be), but in practice, a good C3 sequence would (statistically) turn off 
> >large portions of the CPU and hence the TLB as well.
> >
> 
> I think C3 guarantees that the cache contents stay intact, and thus it 
> might make sense in some technology to preserve the TLB as well (being a 
> kind of cache.)
> 
> Otherwise, what you say here of course is absolutely correct.
> 

C3 does not guarantee all cache contents. Infact, atleast on Intel, 
L1 will be almost always flushed. Newer more power efficient CPUs does dynamic
cache sizing [1]

C3 just guarantees that the caches are coherent. That is, if they are intact,
then DMA will keep cache consistent.

Thanks,
Venki

[1] - http://download.intel.com/products/processor/core2duo/mobile_prod_brief.pdf