[RFC PATCH] arm/arm64: KVM: allow the use of THP on 2MB aligned memslots

[Christoffer Dall]

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 at linaro.org>
> Signed-off-by: Marc Zyngier <marc.zyngier at arm.com>
> ---
>  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(&current->mm->mmap_sem);
> -- 
> 1.8.2.3
> 
> 

-- 
Christoffer