Re: [PATCH v7 24/27] x86: Add swiotlb force off support

[Konrad Rzeszutek Wilk]

On Fri, Dec 21, 2012 at 07:23:03PM -0800, Eric W. Biederman wrote:
> Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx> writes:
> 
> > On Fri, Dec 21, 2012 at 06:42:47PM -0800, Eric W. Biederman wrote:
> >> Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx> writes:
> >> 
> >> > On Mon, Dec 17, 2012 at 11:15:56PM -0800, Yinghai Lu wrote:
> >> >> So use could disable swiotlb from command line, even swiotlb support
> >> >> is compiled in.  Just like we have intel_iommu=on and intel_iommu=off.
> >> >
> >> > You really need to spell out why this is useful.
> >> 
> >> YH why can't we safely autodetect that the swiotlb is unusable when
> >> there is no memory below 4G free?
> >
> > I am not sure what 'YH' stands for (Yeah?).  
> 
> Yinghai Lu's nickname.
> 
> > However we could turn SWIOTLB off altogether if it cannot allocate
> > _some_ memory. It could try first 64MB, then 32MB, lastly 16MB. And
> > if all that fails - print a nice warning and continue on.
> >
> > Later in the late initialization phase, when pci_swiotlb_late_init
> > is called - it can then figure out whether 'iommu' has been set
> > and it iself was never able to allocate. At that point it can try
> > the dynamic allocation (swiotlb_late_init_with_default_size)
> > ... and if that fails give up and panic.
> 
> As far as I can tell panics should be avoided unless there is something
> that actually needs an iommu, and the swiommu is the only option, and
> the swiommu can not fulfill that request.

Right.
> 
> In this case YH has been working on the case of loading a kernel
> completely above 4G, and apparently he has also been testing the case of
> running a kernel with no memory below 4G.
> 
> Eric
> 
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