On Fri, Dec 13, 2013 at 04:56:06PM +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.
>
> It can also choose to place this memslot at whatever alignment it
> wants in the IPA space. In the end, what matters is the relative
> alignment of the user space and IPA mappings with respect to a
> 2M page. They absolutely must be the same if you want to use THP.
Looks good to me:
Acked-by: Christoffer Dall <christoffer.dall@xxxxxxxxxx>
>
> Cc: Christoffer Dall <christoffer.dall@xxxxxxxxxx>
> Signed-off-by: Marc Zyngier <marc.zyngier@xxxxxxx>
> ---
> arch/arm/kvm/mmu.c | 16 +++++++++-------
> 1 file changed, 9 insertions(+), 7 deletions(-)
>
> diff --git a/arch/arm/kvm/mmu.c b/arch/arm/kvm/mmu.c
> index 5809069..9cad7a9 100644
> --- a/arch/arm/kvm/mmu.c
> +++ b/arch/arm/kvm/mmu.c
> @@ -667,14 +667,16 @@ 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
> - * 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.
> + * Pages belonging to memslots that don't have the same
> + * alignment for userspace and IPA 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) !=
> + ((memslot->base_gfn << PAGE_SHIFT) & ~PMD_MASK))
> force_pte = true;
> }
> up_read(¤t->mm->mmap_sem);
> --
> 1.8.2.3
>
>
--
Christoffer
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