Re: [RFC PATCH 2/7] KVM: arm64: Set DBM bit of PTEs if hw DBM enabled

[zhukeqian]

Hi Catalin,

On 2020/5/26 19:49, Catalin Marinas wrote:
> On Mon, May 25, 2020 at 07:24:01PM +0800, Keqian Zhu wrote:
>> diff --git a/arch/arm64/include/asm/pgtable-prot.h b/arch/arm64/include/asm/pgtable-prot.h
>> index 1305e28225fc..f9910ba2afd8 100644
>> --- a/arch/arm64/include/asm/pgtable-prot.h
>> +++ b/arch/arm64/include/asm/pgtable-prot.h
>> @@ -79,6 +79,7 @@ extern bool arm64_use_ng_mappings;
>>  	})
>>  
>>  #define PAGE_S2			__pgprot(_PROT_DEFAULT | PAGE_S2_MEMATTR(NORMAL) | PTE_S2_RDONLY | PAGE_S2_XN)
>> +#define PAGE_S2_DBM		__pgprot(_PROT_DEFAULT | PAGE_S2_MEMATTR(NORMAL) | PTE_S2_RDONLY | PAGE_S2_XN | PTE_DBM)
> 
> You don't need a new page permission (see below).
> 
>>  #define PAGE_S2_DEVICE		__pgprot(_PROT_DEFAULT | PAGE_S2_MEMATTR(DEVICE_nGnRE) | PTE_S2_RDONLY | PTE_S2_XN)
>>  
>>  #define PAGE_NONE		__pgprot(((_PAGE_DEFAULT) & ~PTE_VALID) | PTE_PROT_NONE | PTE_RDONLY | PTE_NG | PTE_PXN | PTE_UXN)
>> diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c
>> index e3b9ee268823..dc97988eb2e0 100644
>> --- a/virt/kvm/arm/mmu.c
>> +++ b/virt/kvm/arm/mmu.c
>> @@ -1426,6 +1426,10 @@ static void stage2_wp_ptes(pmd_t *pmd, phys_addr_t addr, phys_addr_t end)
>>  	pte = pte_offset_kernel(pmd, addr);
>>  	do {
>>  		if (!pte_none(*pte)) {
>> +#ifdef CONFIG_ARM64_HW_AFDBM
>> +			if (kvm_hw_dbm_enabled() && !kvm_s2pte_dbm(pte))
>> +				kvm_set_s2pte_dbm(pte);
>> +#endif
>>  			if (!kvm_s2pte_readonly(pte))
>>  				kvm_set_s2pte_readonly(pte);
>>  		}
> 
> Setting the DBM bit is equivalent to marking the page writable. The
> actual writable pte bit (S2AP[1] or HAP[2] as we call them in Linux for
> legacy reasons) tells you whether the page has been dirtied but it is
> still writable if you set DBM. Doing this in stage2_wp_ptes()
> practically means that you no longer have read-only pages at S2. There
> are several good reasons why you don't want to break this. For example,
> the S2 pte may already be read-only for other reasons (CoW).
> 
Thanks, your comments help to solve the first problem in cover letter.

> I think you should only set the DBM bit if the pte was previously
> writable. In addition, any permission change to the S2 pte must take
> into account the DBM bit and clear it while transferring the dirty
> status to the underlying page. I'm not deeply familiar with all these
> callbacks into KVM but two such paths are kvm_unmap_hva_range() and the
> kvm_mmu_notifier_change_pte().
Yes, I agree.
> 
> 
>> @@ -1827,7 +1831,15 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>>  
>>  		ret = stage2_set_pmd_huge(kvm, memcache, fault_ipa, &new_pmd);
>>  	} else {
>> -		pte_t new_pte = kvm_pfn_pte(pfn, mem_type);
>> +		pte_t new_pte;
>> +
>> +#ifdef CONFIG_ARM64_HW_AFDBM
>> +		if (kvm_hw_dbm_enabled() &&
>> +		    pgprot_val(mem_type) == pgprot_val(PAGE_S2)) {
>> +			mem_type = PAGE_S2_DBM;
>> +		}
>> +#endif
>> +		new_pte = kvm_pfn_pte(pfn, mem_type);
>>  
>>  		if (writable) {
>>  			new_pte = kvm_s2pte_mkwrite(new_pte);
> 
> That's wrong here. Basically for any fault you get, you just turn the S2
> page writable. The point of DBM is that you don't get write faults at
> all if you have a writable page. So, as I said above, only set the DBM
> bit if you stored a writable S2 pte (kvm_s2pte_mkwrite()).
Yeah, you are right. I will correct it in Patch v1.
> 

Thanks,
Keqian