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

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Date:	Tue, 29 Apr 2008 15:14:23 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Amit Shah <amit.shah@...ranet.com>
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
Subject: Re: [PATCH] x86 DMA: Handle devices assigned to the guest by the host

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) 

-Andi

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