linux-kernel - Re: swiotlb detection should be memory hotplug aware ?

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Message-Id: <20100722090329O.fujita.tomonori@lab.ntt.co.jp>
Date:	Thu, 22 Jul 2010 09:03:57 +0900
From:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
To:	akataria@...are.com
Cc:	konrad.wilk@...cle.com, ak@...ux.intel.com, lenb@...nel.org,
	x86@...nel.org, linux-acpi@...r.kernel.org,
	linux-kernel@...r.kernel.org, petr@...are.com
Subject: Re: swiotlb detection should be memory hotplug aware ?

On Thu, 22 Jul 2010 08:44:42 +0900
FUJITA Tomonori <fujita.tomonori@....ntt.co.jp> wrote:

> On Wed, 21 Jul 2010 10:13:34 -0700
> Alok Kataria <akataria@...are.com> wrote:
> 
> > > Basically, you want to add hot-plug memory and enable swiotlb, right?
> > 
> > Not really, I am planning to do something like this, 
> > 
> > @@ -52,7 +52,7 @@ int __init pci_swiotlb_detect(void)
> >  
> >  	/* don't initialize swiotlb if iommu=off (no_iommu=1) */
> >  #ifdef CONFIG_X86_64
> > -	if (!no_iommu && max_pfn > MAX_DMA32_PFN)
> > +	if (!no_iommu && (max_pfn > MAX_DMA32_PFN || hotplug_possible()))
> >  		swiotlb = 1;
> 
> Always enable swiotlb with memory hotplug enabled? Wasting 64MB on a
> x86_64 system with 128MB doesn't look to be a good idea. I don't think
> that there is an easy solution for this issue though.

btw, you need more work to enable switch on the fly.

You need to change the dma_ops pointer (see get_dma_ops()). It means
that you need to track outstanding dma operations per device, locking,
etc.
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