linux-kernel - Re: [PATCH v2 3/3] KVM: remove dummy pages

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Message-ID: <50110F51.3050903@linux.vnet.ibm.com>
Date:	Thu, 26 Jul 2012 17:35:13 +0800
From:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
To:	Takuya Yoshikawa <yoshikawa.takuya@....ntt.co.jp>
CC:	Avi Kivity <avi@...hat.com>, Marcelo Tosatti <mtosatti@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, KVM <kvm@...r.kernel.org>
Subject: Re: [PATCH v2 3/3] KVM: remove dummy pages

On 07/26/2012 05:20 PM, Takuya Yoshikawa wrote:
> On Thu, 26 Jul 2012 11:56:15 +0300
> Avi Kivity <avi@...hat.com> wrote:
> 
>> Since my comments are better done as a separate patch, I applied all
>> three patches.  Thanks!
> 
> Is this patch really safe for all architectures?
> 
> IS_ERR_VALUE() casts -MAX_ERRNO to unsigned long and then does comparison.
> Isn't it possible to conflict with valid pfns?
> 

See IS_ERR_VALUE():

#define IS_ERR_VALUE(x) unlikely((x) >= (unsigned long)-MAX_ERRNO)

The minimal value of the error code is:
0xffff f001 on 32-bit and 0x ffff ffff ffff f001 on 64-bit,
it is fair larger that a valid pfn (for the pfn, the most top of 12 bits
are always 0).

Note, PAE is a special case, but only 64G physical memory is valid,
0xffff f001 is also suitable for that.

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