Re: [PATCH 3/3] arm, arm64: KVM: handle potential incoherency of readonly memslots

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On 17 November 2014 15:58, Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx> wrote:
> Readonly memslots are often used to implement emulation of ROMs and
> NOR flashes, in which case the guest may legally map these regions as
> uncached.
> To deal with the incoherency associated with uncached guest mappings,
> treat all readonly memslots as incoherent, and ensure that pages that
> belong to regions tagged as such are flushed to DRAM before being passed
> to the guest.
>
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
> ---

Hello all,

I have another bug report (from Canonical this time) of essentially
the same issue, and it is also fixed by these patches.
Are you happy with these patches? Should I respin to add Laszlo's tested-by?

Cheers,
Ard.


>  arch/arm/kvm/mmu.c | 20 +++++++++++++++-----
>  1 file changed, 15 insertions(+), 5 deletions(-)
>
> diff --git a/arch/arm/kvm/mmu.c b/arch/arm/kvm/mmu.c
> index cb924c6d56a6..f2a9874ff5cb 100644
> --- a/arch/arm/kvm/mmu.c
> +++ b/arch/arm/kvm/mmu.c
> @@ -919,7 +919,7 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>         if (!hugetlb && !force_pte)
>                 hugetlb = transparent_hugepage_adjust(&pfn, &fault_ipa);
>
> -       fault_ipa_uncached = false;
> +       fault_ipa_uncached = memslot->flags & KVM_MEMSLOT_INCOHERENT;
>
>         if (hugetlb) {
>                 pmd_t new_pmd = pfn_pmd(pfn, mem_type);
> @@ -1298,11 +1298,12 @@ int kvm_arch_prepare_memory_region(struct kvm *kvm,
>                 hva = vm_end;
>         } while (hva < reg_end);
>
> -       if (ret) {
> -               spin_lock(&kvm->mmu_lock);
> +       spin_lock(&kvm->mmu_lock);
> +       if (ret)
>                 unmap_stage2_range(kvm, mem->guest_phys_addr, mem->memory_size);
> -               spin_unlock(&kvm->mmu_lock);
> -       }
> +       else
> +               stage2_flush_memslot(kvm, memslot);
> +       spin_unlock(&kvm->mmu_lock);
>         return ret;
>  }
>
> @@ -1314,6 +1315,15 @@ void kvm_arch_free_memslot(struct kvm *kvm, struct kvm_memory_slot *free,
>  int kvm_arch_create_memslot(struct kvm *kvm, struct kvm_memory_slot *slot,
>                             unsigned long npages)
>  {
> +       /*
> +        * Readonly memslots are not incoherent with the caches by definition,
> +        * but in practice, they are used mostly to emulate ROMs or NOR flashes
> +        * that the guest may consider devices and hence map as uncached.
> +        * To prevent incoherency issues in these cases, tag all readonly
> +        * regions as incoherent.
> +        */
> +       if (slot->flags & KVM_MEM_READONLY)
> +               slot->flags |= KVM_MEMSLOT_INCOHERENT;
>         return 0;
>  }
>
> --
> 1.8.3.2
>
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