linux-kernel - Re: [PATCH] x86 DMA: Handle devices assigned to the guest by the host

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Date:	Tue, 29 Apr 2008 19:19:23 +0530
From:	Amit Shah <amit.shah@...ranet.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	kvm-devel@...ts.sourceforge.net, chrisw@...hat.com,
	allen.m.kay@...el.com, linux-kernel@...r.kernel.org,
	gcosta@...hat.com, avi@...ranet.com,
	virtualization@...ts.linux-foundation.org, BENAMI@...ibm.com,
	"Muli Ben-Yehuda" <muli@...ibm.com>
Subject: Re: [PATCH] x86 DMA: Handle devices assigned to the guest by the host

On Tuesday 29 April 2008 18:44:23 Andi Kleen wrote:
> Amit Shah <amit.shah@...ranet.com> writes:
> > diff --git a/arch/x86/kernel/pci-dma.c b/arch/x86/kernel/pci-dma.c
> > index 388b113..678cafb 100644
> > --- a/arch/x86/kernel/pci-dma.c
> > +++ b/arch/x86/kernel/pci-dma.c
> > @@ -443,6 +443,17 @@ dma_alloc_coherent(struct device *dev, size_t size,
> > dma_addr_t *dma_handle, memset(memory, 0, size);
> >  		if (!mmu) {
> >  			*dma_handle = bus;
> > +			if (unlikely(dma_ops->is_pv_device) &&
> > +			    unlikely(dma_ops->is_pv_device(dev, dev->bus_id))) {
>
> First double unlikely in a condition is useless. Just drop them.
>
> And then ->is_xyz() in a generic vops interface is about as ugly
> and non generic as you can get. dma_alloc_coherent is not performance
> critical, so you should rather change the interface that ->alloc_coherent
> is always called and the other handlers handle the !mmu case correctly.
> In fact they need that already I guess (e.g. on DMAR there is not really
> a nommu case)

This point came up the last time I sent out the patch; we should do this as 
well as implement stackable dma_ops (the need for that is evident in the next 
patch).

Thanks for the observation; this should be the next step.

Amit.
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