linux-kernel - Re: [PATCH v3 2/9] KVM: MMU: abstract spte write-protect

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Date:	Sat, 21 Apr 2012 14:52:21 +0800
From:	Xiao Guangrong <xiaoguangrong.eric@...il.com>
To:	Marcelo Tosatti <mtosatti@...hat.com>
CC:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>,
	Avi Kivity <avi@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, KVM <kvm@...r.kernel.org>
Subject: Re: [PATCH v3 2/9] KVM: MMU: abstract spte write-protect

On 04/21/2012 12:18 PM, Marcelo Tosatti wrote:

> On Sat, Apr 21, 2012 at 11:24:54AM +0800, Xiao Guangrong wrote:
>> On 04/21/2012 05:33 AM, Marcelo Tosatti wrote:
>>
>>
>>>>  static bool
>>>>  __rmap_write_protect(struct kvm *kvm, unsigned long *rmapp, int level)
>>>>  {
>>>> @@ -1050,24 +1078,13 @@ __rmap_write_protect(struct kvm *kvm, unsigned long *rmapp, int level)
>>>>
>>>>  	for (sptep = rmap_get_first(*rmapp, &iter); sptep;) {
>>>>  		BUG_ON(!(*sptep & PT_PRESENT_MASK));
>>>> -		rmap_printk("rmap_write_protect: spte %p %llx\n", sptep, *sptep);
>>>> -
>>>> -		if (!is_writable_pte(*sptep)) {
>>>> -			sptep = rmap_get_next(&iter);
>>>> -			continue;
>>>> -		}
>>>> -
>>>> -		if (level == PT_PAGE_TABLE_LEVEL) {
>>>> -			mmu_spte_update(sptep, *sptep & ~PT_WRITABLE_MASK);
>>>> -			sptep = rmap_get_next(&iter);
>>>> -		} else {
>>>> -			BUG_ON(!is_large_pte(*sptep));
>>>> -			drop_spte(kvm, sptep);
>>>> -			--kvm->stat.lpages;
>>>
>>> It is preferable to remove all large sptes including read-only ones, the
>>
>>
>> It can cause page faults even if read memory on these large sptse.
>>
>> Actually, Avi suggested that make large writable spte to be readonly
>> (not dropped) on this path.
> 
> See commits e49146dce8c3dc6f4485c1904b6587855f393e71,
> 38187c830cab84daecb41169948467f1f19317e3 for issues
> with large read-only sptes.
> 


Yes, we need check the code carefully when change writable spte to be
read-only, let us discuss it in the separate patchset later. :)

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