Re: [PATCH v7u1 26/31] x86: Don't enable swiotlb if there is notenough ram for it

From: Yinghai Lu
Date: Fri Jan 04 2013 - 17:58:07 EST

On Fri, Jan 4, 2013 at 2:47 PM, Eric W. Biederman <ebiederm@xxxxxxxxxxxx> 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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