Subject: [PATCH RESEND v15 07/10] KVM: arm: page logging 2nd stage fault handling

From: [email-masked] (Christoffer Dall)
Date: Wed, 14 Jan 2015 11:32:33 +0100

On Tue, Jan 13, 2015 at 03:14:47PM -0800, Mario Smarduch wrote:

[...]

> >>>
> >>>
> >>> What I meant last time around concerning user_mem_abort was more
> >>> something like this:
> >>>
> >>> diff --git a/arch/arm/kvm/mmu.c b/arch/arm/kvm/mmu.c
> >>> index 1dc9778..38ea58e 100644
> >>> --- a/arch/arm/kvm/mmu.c
> >>> +++ b/arch/arm/kvm/mmu.c
> >>> @@ -935,7 +935,14 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
> >>>  		return -EFAULT;
> >>>  	}
> >>>  
> >>> -	if (is_vm_hugetlb_page(vma)) {
> >>> +	/*
> >>> +	 * Writes to pages in a memslot with logging enabled are always logged
> >>> +	 * on a singe page-by-page basis.
> >>> +	 */
> >>> +	if (memslot_is_logging(memslot) && write_fault)
> >>> +		force_pte = true;
> >>
> >> If it's a write you take the pte route and
> >> dissolves huge page, if it's a read you reconstruct the
> >> THP that seems to yield pretty bad results.
> > 
> > ok, then remove the ' && write_fault' part of the clause.
> Hi Christoffer,
>  couple comments/questions.
> 
>  setting force_pte here, disables huge pages for
> non-writable regions.
> 

hmmm, by a non-writable region you mean a read-only memslot? Can you set
dirty page logging for such one?  That doesn't make much sense to me.

Note, that if you receive writable == false from gfn_to_pfn_prot() that
doesn't mean that the page can never be written to, it just means that
the current mapping of the page is not a writable one, you can call that
same function again later with write_fault=true, and you either get a
writable page back or you simply get an error.

[...]

> >>>  	if (kvm_is_device_pfn(pfn))
> >>>  		mem_type = PAGE_S2_DEVICE;
> 
> If we're not setting the IOMAP flag do we have need
> this, since we're forfeiting error checking later
> in stage2_set_pte()?
> 

we still need this, remember the error checking is about
cache == NULL, not about the IOMAP flag.  I think I address this in the
new proposal below, but please check carefully.

Take a look at this one:

diff --git a/arch/arm/kvm/mmu.c b/arch/arm/kvm/mmu.c
index 1dc9778..841e053 100644
--- a/arch/arm/kvm/mmu.c
+++ b/arch/arm/kvm/mmu.c
@@ -919,6 +919,7 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
 	pfn_t pfn;
 	pgprot_t mem_type = PAGE_S2;
 	bool fault_ipa_uncached;
+	unsigned long flags = 0;
 
 	write_fault = kvm_is_write_fault(vcpu);
 	if (fault_status == FSC_PERM && !write_fault) {
@@ -976,8 +977,26 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
 	if (is_error_pfn(pfn))
 		return -EFAULT;
 
-	if (kvm_is_device_pfn(pfn))
+	if (kvm_is_device_pfn(pfn)) {
 		mem_type = PAGE_S2_DEVICE;
+		flags |= KVM_S2PTE_FLAG_IS_IOMAP;
+	} else if (memslot_is_logging(memslot)) {
+		/*
+		 * Faults on pages in a memslot with logging enabled that can
+		 * should not be mapped with huge pages (it introduces churn
+		 * and performance degradation), so force a pte mapping.
+		 */
+		force_pte = true;
+		flags |= KVM_S2_FLAG_LOGGING_ACTIVE;
+
+		/*
+		 * Only actually map the page as writable if this was a write
+		 * fault.
+		 */
+		if (!write_fault)
+			writable = false;
+
+	}
 
 	spin_lock(&kvm->mmu_lock);
 	if (mmu_notifier_retry(kvm, mmu_seq))
@@ -1002,13 +1021,13 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
 		if (writable) {
 			kvm_set_s2pte_writable(&new_pte);
 			kvm_set_pfn_dirty(pfn);
+			mark_page_dirty(kvm, gfn);
 		}
 		coherent_cache_guest_page(vcpu, hva, PAGE_SIZE,
 					  fault_ipa_uncached);
-		ret = stage2_set_pte(kvm, memcache, fault_ipa, &new_pte,
-			pgprot_val(mem_type) == pgprot_val(PAGE_S2_DEVICE));
-	}
 
+		ret = stage2_set_pte(kvm, memcache, fault_ipa, &new_pte, flags);
+	}
 
 out_unlock:
 	spin_unlock(&kvm->mmu_lock);

Thanks,
-Christoffer