linux-kernel - Re: [PATCH 11/22] KVM: MMU: Add infrastructure for two-level page walker

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Date:	Wed, 28 Apr 2010 14:24:16 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Joerg Roedel <joerg.roedel@....com>
CC:	Marcelo Tosatti <mtosatti@...hat.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 11/22] KVM: MMU: Add infrastructure for two-level page
 walker

On 04/28/2010 01:52 PM, Joerg Roedel wrote:
> On Tue, Apr 27, 2010 at 03:34:10PM +0300, Avi Kivity wrote:
>    
>> On 04/27/2010 01:38 PM, Joerg Roedel wrote:
>>      
>>> This patch introduces a mmu-callback to translate gpa
>>> addresses in the walk_addr code. This is later used to
>>> translate l2_gpa addresses into l1_gpa addresses.
>>>
>>> +static inline gfn_t gpa_to_gfn(gpa_t gpa)
>>> +{
>>> +	return (gfn_t)gpa>>   PAGE_SHIFT;
>>> +}
>>> +
>>>        
>> This overflows on 32-bit, since gpa_t is u64 and gfn_t is ulong.
>>      
> Hm, this is a problem outside of this patchset too (for 32bit hosts).
> The best solution is probably to convert gfn_t to u64 too.
>    

If you cast like

    (gfn_t)(gpa >> PAGE_SHIFT)

you avoid the overflow for MAXPHYADDR < 48.  However, I agree that 
converting gfn_t to u64 is best, the minor performance degradation is in 
no way comparable to the corruption that results from a miscast.

-- 
error compiling committee.c: too many arguments to function

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