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

Gleb Natapov
2010-10-05T13:43:03Z

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.