Re: [patch 2/3] KVM: dont require read-only host ptes

[Gleb Natapov <gleb@redhat.com>]

On Tue, Oct 05, 2010 at 08:55:00AM -0300, Marcelo Tosatti wrote:
> gfn_to_pfn requires a writable host pte, failing otherwise.
> 
> Change it to fallback to read-only "acquision', informing the callers. 
> 
> Hopefully the ptes are cache-hot so the overhead is minimal.
> 
> Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
> 
> Index: kvm/arch/ia64/kvm/kvm-ia64.c
> ===================================================================
> --- kvm.orig/arch/ia64/kvm/kvm-ia64.c
> +++ kvm/arch/ia64/kvm/kvm-ia64.c
> @@ -1589,7 +1589,7 @@ int kvm_arch_prepare_memory_region(struc
>  		return -ENOMEM;
>  
>  	for (i = 0; i < npages; i++) {
> -		pfn = gfn_to_pfn(kvm, base_gfn + i);
> +		pfn = gfn_to_pfn(kvm, base_gfn + i, NULL);
>  		if (!kvm_is_mmio_pfn(pfn)) {
>  			kvm_set_pmt_entry(kvm, base_gfn + i,
>  					pfn << PAGE_SHIFT,
> Index: kvm/arch/x86/kvm/mmu.c
> ===================================================================
> --- kvm.orig/arch/x86/kvm/mmu.c
> +++ kvm/arch/x86/kvm/mmu.c
> @@ -2273,6 +2273,7 @@ static int nonpaging_map(struct kvm_vcpu
>  {
>  	int r;
>  	int level;
> +	int writable;
>  	pfn_t pfn;
>  	unsigned long mmu_seq;
>  
> @@ -2289,10 +2290,10 @@ static int nonpaging_map(struct kvm_vcpu
>  
>  	mmu_seq = vcpu->kvm->mmu_notifier_seq;
>  	smp_rmb();
> -	pfn = gfn_to_pfn(vcpu->kvm, gfn);
> +	pfn = gfn_to_pfn(vcpu->kvm, gfn, &writable);
>  
>  	/* mmio */
> -	if (is_error_pfn(pfn))
> +	if (is_error_pfn(pfn) || !writable)
>  		return kvm_handle_bad_page(vcpu->kvm, gfn, pfn);
>  
>  	spin_lock(&vcpu->kvm->mmu_lock);
> @@ -2581,6 +2582,8 @@ static int tdp_page_fault(struct kvm_vcp
>  	pfn_t pfn;
>  	int r;
>  	int level;
> +	int writable;
> +	int write = error_code & PFERR_WRITE_MASK;
>  	gfn_t gfn = gpa >> PAGE_SHIFT;
>  	unsigned long mmu_seq;
>  
> @@ -2597,15 +2600,14 @@ static int tdp_page_fault(struct kvm_vcp
>  
>  	mmu_seq = vcpu->kvm->mmu_notifier_seq;
>  	smp_rmb();
> -	pfn = gfn_to_pfn(vcpu->kvm, gfn);
> -	if (is_error_pfn(pfn))
> +	pfn = gfn_to_pfn(vcpu->kvm, gfn, &writable);
> +	if (is_error_pfn(pfn) || !writable)
Why would we fail read only access to read only memory? Shouldn't we
check access type here?

--
			Gleb.