Re: [Xen-devel] [PATCH 4/7] xen/arm: set the SMP bit in the ACTLR register

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Fri, 2012-10-26 at 09:56 +0100, Tim Deegan wrote:
> At 13:04 +0100 on 25 Oct (1351170270), Ian Campbell wrote:
> > On Thu, 2012-10-25 at 12:57 +0100, Stefano Stabellini wrote:
> > > On Thu, 25 Oct 2012, Ian Campbell wrote:
> > > > On Wed, 2012-10-24 at 16:03 +0100, Stefano Stabellini wrote:
> > > > > From the Cortex A15 manual:
> > > > > 
> > > > > "Enables the processor to receive instruction cache, BTB, and TLB 
> > > > > maintenance
> > > > > operations from other processors"
> > > > > 
> > > > > ...
> > > > > 
> > > > > "You must set this bit before enabling the caches and MMU, or
> > > > > performing any cache and TLB maintenance operations. The only time
> > > > > you must clear this bit is during a processor power-down sequence"
> > > > 
> > > > Is it considered a bug that the firmware doesn't do this?
> > > 
> > > Why would it be? You can run fairly complicated pieces of software
> > > without caches or MMU.
> > 
> > True, I guess I just considered that not setting the SMP bit on the
> > second processor when the firmware starts it seemed a bit odd. If you
> > aren't caches/MMU/etc them then setting the bit is pretty much a NOP (or
> > maybe it isn't?).
> 
> Well, given that you're not allowed to clear it except at power-down, I
> think it would be impolite of the firmware to set it if there's _any_
> reason the OS might not want it set.

True.



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