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

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Date:	Fri, 04 Jun 2010 16:14:08 +0800
From:	Lai Jiangshan <laijs@...fujitsu.com>
To:	Avi Kivity <avi@...hat.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

Avi Kivity wrote:
> On 06/02/2010 11:53 AM, Lai Jiangshan wrote:
>> Current code uses slot_bitmap to find ptes who map a page
>> from the memory slot, it is not precise: some ptes in the shadow page
>> are not map any page from the memory slot.
>>
>> This patch uses rmap to find the ptes precisely, and remove
>> the unused slot_bitmap.
>>
>>    
> 
> Patch looks good; a couple of comments:
> 
> - We might see a slowdown with !tdp, since we no longer have locality. 
> Each page will map to an spte in a different page.  However, it's still
> worth it in my opinion.

Yes, this patch hurts the cache since we no longer have locality.
And if most pages of the slot are not mapped(rmap_next(kvm, rmapp, NULL)==NULL),
this patch will worse than old method I think.

This patch do things straightly, precisely.

> - 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?

> 
>>
>> +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.


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