linux-kernel - Re: [PATCH 0/3] Early use of boot service memory

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Date:	Fri, 15 Nov 2013 09:40:49 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Yinghai Lu <yinghai@...nel.org>, Vivek Goyal <vgoyal@...hat.com>
CC:	Ingo Molnar <mingo@...nel.org>, jerry.hoemann@...com,
	Pekka Enberg <penberg@...nel.org>,
	Rob Landley <rob@...dley.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	x86 maintainers <x86@...nel.org>,
	Matt Fleming <matt.fleming@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"list@...ederm.org:DOCUMENTATION" <linux-doc@...r.kernel.org>,
	"list@...ederm.org:MEMORY MANAGEMENT" <linux-mm@...ck.org>,
	linux-efi@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/3] Early use of boot service memory

On 11/15/2013 09:33 AM, Yinghai Lu wrote:
> 
> If the system support intel IOMMU, we only need to that 72M for SWIOTLB
> or AMD workaround.
> If the user really care that for intel iommu enable system, they could use
> "crashkernel=0,low" to have that 72M back.
> 
> and that 72M is under 4G instead of 896M.
> 
> so reserve 72M is not better than reserve 128M?
> 

Those 72M are in addition to 128M, which does add up quite a bit.
However, the presence of a working IOMMU in the system is something that
should be possible to know at setup time.

Now, this was discussed partly in the context of VMs.  I want to say, as
I have again and again: the right way to dump a VM is with hypervisor
assistance rather than an in-image dumper which is both expensive and
may be corrupted by the failure.

It would be good if the various VMs with interest in Linux would agree
on a mechanism for launching a dumper.  This can be done either inband
(on the execution of a specific hypercall, the hypervisor terminates I/O
to the guest, inserts a dumper into the address space and launches it)
or out-of-band (the hypervisor itself, or an assistant program, writes a
dump file) or as a hybrid (a new dump guest is launched with the
hypervisor-written or hypervisor-preserved crashed guest image somehow
passed to it.)

	-hpa

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