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Date: Mon, 8 Apr 2013 11:37:06 -0700
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Subject: Re: [PATCH v3 1/4] x86, kdump: Set crashkernel_low automatically
From: Yinghai Lu <yinghai@kernel.org>
To: Dave Young <dyoung@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@elte.hu>,
        "H. Peter Anvin" <hpa@zytor.com>, WANG Chao <chaowang@redhat.com>,
        Vivek Goyal <vgoyal@redhat.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Mon, Apr 8, 2013 at 12:09 AM, Dave Young <dyoung@redhat.com> wrote:
>> We have now modified crashkernel=X to allocate memory beyong 4G (if
>> available) and do not allocate low range for crashkernel if the user
>> does not specify that with crashkernel_low=Y.  This causes regression
>> if iommu is not enabled.  Without iommu, swiotlb needs to be setup in
>> first 4G and there is no low memory available to second kernel.
>
> Is it possible to reuse the 1st kernel swiotlb region in 2nd capture
> kernel if it's available?

If the first kernel is using intel iommu, and swiotlb is freed after intel
iommus is enabled in first kernel.

>
>>
>> Set crashkernel_low automatically if the user does not specify that.
>>
>> For system that does support IOMMU with kdump properly, user could
>> specify crashkernel_low=0 to save that 72M low ram.
>
> How about make swiotlb size tunable in 1st kernel as well such as adding
> a swiotlb_size= to cmdline, if it's set in 1st kernel crashkernel
> reserving code can take it automaticlly.
>
can not understand this.

Thanks

Yinghai