linux-kernel - Re: [PATCH v7u1 26/31] x86: Don't enable swiotlb if there is not enough ram for it

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Date:	Fri, 4 Jan 2013 14:58:15 -0800
From:	Yinghai Lu <yinghai@...nel.org>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Shuah Khan <shuahkhan@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Borislav Petkov <bp@...en8.de>, Jan Kiszka <jan.kiszka@....de>,
	Jason Wessel <jason.wessel@...driver.com>,
	linux-kernel@...r.kernel.org,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH v7u1 26/31] x86: Don't enable swiotlb if there is not
 enough ram for it

On Fri, Jan 4, 2013 at 2:47 PM, Eric W. Biederman <ebiederm@...ssion.com> wrote:
> Yinghai Lu it looks like your autodetection of the problem case in this
> patch is problematic and needs a rethink.  My quick skim says you are
> trying to detect failure too early in the code.  Furthermore having
> kexec on panic sized magic comments without explanation is wrong.

current amd iommu implementation have this sequence:
1. alloc buffer for swiotlb.
2. detect and initialize intel iommu or amd iommu
3. release swiotlb if swiotlb == 0 , set by ops_init.

so we need to detect that before allocating buffer for swiotlb.

>
> Shuah Khan this is motivated by kdump.  However a correct implementation
> should be about dealing with the case when there is simply not enough
> memory available below 4G for bounce buffers.
>
> If a device needs an iommu, and swiotlb is the only iommu option, and
> there is not enough memory below 4G panic'ing is entirely reasonable.
>
> Do I read this discussion right that we are waisting 64M on systems
> that have the swiotlb code but don't use the swiotlb?

No wasting.
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