On Thu, Dec 12, 2013 at 02:51:21PM +0000, Marc Zyngier wrote:
> The THP code in KVM/ARM is a bit restrictive in not allowing a THP
> to be used if the VMA is not 2MB aligned. Actually, it is not so much
> the VMA that matters, but the associated memslot:
>
> A process can perfectly mmap a region with no particular alignment
> restriction, and then pass a 2MB aligned address to KVM. In this
> case, KVM will only use this 2MB aligned region, and will ignore
> the range between vma->vm_start and memslot->userspace_addr.
>
> The fix is then to check the alignment of memslot->userspace_addr.
That's more correct, but I'm wondering if it's enough.
What happens if the base_gfn is not aligned to a 2MB region, will we not
be mapping something completely bogus here?
>
> Cc: Christoffer Dall <christoffer.dall@xxxxxxxxxx>
> Signed-off-by: Marc Zyngier <marc.zyngier@xxxxxxx>
> ---
> arch/arm/kvm/mmu.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arm/kvm/mmu.c b/arch/arm/kvm/mmu.c
> index 5809069..cec641a 100644
> --- a/arch/arm/kvm/mmu.c
> +++ b/arch/arm/kvm/mmu.c
> @@ -667,14 +667,14 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
> gfn = (fault_ipa & PMD_MASK) >> PAGE_SHIFT;
> } else {
> /*
> - * Pages belonging to VMAs not aligned to the PMD mapping
> + * Pages belonging to memslots not aligned to the PMD mapping
> * granularity cannot be mapped using block descriptors even
> * if the pages belong to a THP for the process, because the
> * stage-2 block descriptor will cover more than a single THP
> * and we loose atomicity for unmapping, updates, and splits
> * of the THP or other pages in the stage-2 block range.
> */
> - if (vma->vm_start & ~PMD_MASK)
> + if (memslot->userspace_addr & ~PMD_MASK)
> force_pte = true;
> }
> up_read(¤t->mm->mmap_sem);
> --
> 1.8.2.3
>
>
--
Christoffer
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