linux-kernel - Re: [PATCH 09/11] KVM: MMU: simplify mmu_need_write

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Date:	Tue, 15 Dec 2015 17:25:10 +0800
From:	Xiao Guangrong <guangrong.xiao@...ux.intel.com>
To:	Kai Huang <kai.huang@...ux.intel.com>, pbonzini@...hat.com
Cc:	gleb@...nel.org, mtosatti@...hat.com, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 09/11] KVM: MMU: simplify mmu_need_write_protect



On 12/15/2015 04:43 PM, Kai Huang wrote:
>
>
> On 12/01/2015 02:26 AM, Xiao Guangrong wrote:
>> Now, all non-leaf shadow page are page tracked, if gfn is not tracked
>> there is no non-leaf shadow page of gfn is existed, we can directly
>> make the shadow page of gfn to unsync
>>
>> Signed-off-by: Xiao Guangrong <guangrong.xiao@...ux.intel.com>
>> ---
>>   arch/x86/kvm/mmu.c | 26 ++++++++------------------
>>   1 file changed, 8 insertions(+), 18 deletions(-)
>>
>> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
>> index 5a2ca73..f89e77f 100644
>> --- a/arch/x86/kvm/mmu.c
>> +++ b/arch/x86/kvm/mmu.c
>> @@ -2461,41 +2461,31 @@ static void __kvm_unsync_page(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp)
>>       kvm_mmu_mark_parents_unsync(sp);
>>   }
>> -static void kvm_unsync_pages(struct kvm_vcpu *vcpu,  gfn_t gfn)
>> +static bool kvm_unsync_pages(struct kvm_vcpu *vcpu, gfn_t gfn,
>> +                 bool can_unsync)
>>   {
>>       struct kvm_mmu_page *s;
>>       for_each_gfn_indirect_valid_sp(vcpu->kvm, s, gfn) {
>> +        if (!can_unsync)
>> +            return true;
> How about moving this right before for_each_gfn_indirect_valid_sp? As can_unsync is passed as
> parameter, so there's no point checking it several times.
>

We can not do this. What we are doing here is checking if we have shadow page mapping
for 'gfn':
a) if no, it can be writable.
b) if yes, check 'can_unsync' to see if these shadow pages can make to be 'unsync'.

Your suggestion can break the point a).

> A further thinking is can we move it to mmu_need_write_protect? Passing can_unsync as parameter to
> kvm_unsync_pages sounds a little bit odd.
>
>> +
>>           if (s->unsync)
>>               continue;
>>           WARN_ON(s->role.level != PT_PAGE_TABLE_LEVEL);
> How about large page mapping? Such as if guest uses 2M mapping and its shadow is indirect, does
> above WARN_ON still meet? As you removed the PT level check in mmu_need_write_protect.

The lager mapping are on the non-leaf shadow pages which can be figured out by
kvm_page_track_check_mode() before we call this function.

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