Re: [PATCH 2/5] KVM: MMU: simplify mmu_set_spte

[Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>]

On 11/13/2012 07:12 AM, Marcelo Tosatti wrote:
> On Mon, Nov 05, 2012 at 08:10:08PM +0800, Xiao Guangrong wrote:
>> In order to detecting spte remapping, we can simply check whether the
>> spte has already been pointing to the pfn even if the spte is not the
>> last spte for middle spte is pointing to the kernel pfn which can not
>> be mapped to userspace
>>
>> Also, update slot and stat.lpages iff the spte is not remapped
>>
>> Signed-off-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
>> ---
>>  arch/x86/kvm/mmu.c |   40 +++++++++++++---------------------------
>>  1 files changed, 13 insertions(+), 27 deletions(-)
>>
>> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
>> index 692ebb1..4ea731e 100644
>> --- a/arch/x86/kvm/mmu.c
>> +++ b/arch/x86/kvm/mmu.c
>> @@ -2420,8 +2420,7 @@ static void mmu_set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
>>  			 pfn_t pfn, bool speculative,
>>  			 bool host_writable)
>>  {
>> -	int was_rmapped = 0;
>> -	int rmap_count;
>> +	bool was_rmapped = false;
>>
>>  	pgprintk("%s: spte %llx access %x write_fault %d"
>>  		 " user_fault %d gfn %llx\n",
>> @@ -2429,25 +2428,13 @@ static void mmu_set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
>>  		 write_fault, user_fault, gfn);
>>
>>  	if (is_rmap_spte(*sptep)) {
>> -		/*
>> -		 * If we overwrite a PTE page pointer with a 2MB PMD, unlink
>> -		 * the parent of the now unreachable PTE.
>> -		 */
>> -		if (level > PT_PAGE_TABLE_LEVEL &&
>> -		    !is_large_pte(*sptep)) {
>> -			struct kvm_mmu_page *child;
>> -			u64 pte = *sptep;
>> +		if (pfn != spte_to_pfn(*sptep)) {
>> +			struct kvm_mmu_page *sp = page_header(__pa(sptep));
>>
>> -			child = page_header(pte & PT64_BASE_ADDR_MASK);
>> -			drop_parent_pte(child, sptep);
>> -			kvm_flush_remote_tlbs(vcpu->kvm);
> 
> How come its safe to drop this case?

We use "if (pfn != spte_to_pfn(*sptep))" to simplify the thing.
There are two cases:
1) the sptep is not the last mapping.
   under this case, sptep must point to a shadow page table, that means
   spte_to_pfn(*sptep)) is used by KVM module, and 'pfn' is used by userspace.
   so, 'if' condition must be satisfied, the sptep will be dropped.

   Actually, This is the origin case:
  | if (level > PT_PAGE_TABLE_LEVEL &&
  |	    !is_large_pte(*sptep))"

2) the sptep is the last mapping.
   under this case, the level of spte (sp.level) must equal the 'level' which
   we pass to mmu_set_spte. If they point to the same pfn, it is 'remap', otherwise
   we drop it.

I think this is safe. :)