linux-kernel - Re: [PATCH] kvm: rework remove-write-access for a slot

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Date:	Sun, 06 Jun 2010 19:04:25 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Lai Jiangshan <laijs@...fujitsu.com>
CC:	Marcelo Tosatti <mtosatti@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, kvm@...r.kernel.org
Subject: Re: [PATCH] kvm: rework remove-write-access for a slot

On 06/04/2010 11:14 AM, Lai Jiangshan wrote:
>
>
>> - I thought of a different approach to write protection: write protect
>> the L4 sptes, on write fault add write permission to the L4 spte and
>> write protect the L3 sptes that it points to, etc.  This method can use
>> the slot bitmap to reduce the number of write faults.  However we can
>> reintroduce the slot bitmap if/when we use the method, this shouldn't
>> block the patch.
>>      
> It is very a good approach and it is blazing fast.
>
> I have no time to implement it currently,
> could you update it into the TODO list?
>    

Done.

>>> +static void rmapp_remove_write_access(struct kvm *kvm, unsigned long
>>> *rmapp)
>>> +{
>>> +    u64 *spte = rmap_next(kvm, rmapp, NULL);
>>> +
>>> +    while (spte) {
>>> +        /* avoid RMW */
>>> +        if (is_writable_pte(*spte))
>>> +            *spte&= ~PT_WRITABLE_MASK;
>>>        
>> Must use an atomic operation here to avoid losing dirty or accessed bit.
>>
>>      
> Atomic operation is too expensive, I retained the comment "/* avoid RMW */"
> and wait someone take a good approach for it.
>    

You are right, it is an existing problem.  I just posted a patchset 
which fixes the problem, when it's merged please rebase on top.

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

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