[PATCH 2/2] KVM: arm64: Try PMD block mappings if PUD mappings are not supported

[Alexandru Elisei]

Hi Marc,

On 9/4/20 10:58 AM, Marc Zyngier wrote:
> Hi Alex,
>
> On Tue, 01 Sep 2020 14:33:57 +0100,
> Alexandru Elisei <alexandru.elisei at arm.com> wrote:
>> When userspace uses hugetlbfs for the VM memory, user_mem_abort() tries to
>> use the same block size to map the faulting IPA in stage 2. If stage 2
>> cannot use the same size mapping because the block size doesn't fit in the
>> memslot or the memslot is not properly aligned, user_mem_abort() will fall
>> back to a page mapping, regardless of the block size. We can do better for
>> PUD backed hugetlbfs by checking if a PMD block mapping is possible before
>> deciding to use a page.
>>
>> vma_pagesize is an unsigned long, use 1UL instead of 1ULL when assigning
>> its value.
>>
>> Signed-off-by: Alexandru Elisei <alexandru.elisei at arm.com>
>> ---
>>  arch/arm64/kvm/mmu.c | 19 ++++++++++++++-----
>>  1 file changed, 14 insertions(+), 5 deletions(-)
>>
>> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
>> index 25e7dc52c086..f590f7355cda 100644
>> --- a/arch/arm64/kvm/mmu.c
>> +++ b/arch/arm64/kvm/mmu.c
>> @@ -1871,15 +1871,24 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>>  	else
>>  		vma_shift = PAGE_SHIFT;
>>  
>> -	vma_pagesize = 1ULL << vma_shift;
>>  	if (logging_active ||
>> -	    (vma->vm_flags & VM_PFNMAP) ||
>> -	    !fault_supports_stage2_huge_mapping(memslot, hva, vma_pagesize)) {
>> +	    (vma->vm_flags & VM_PFNMAP)) {
>>  		force_pte = true;
>> -		vma_pagesize = PAGE_SIZE;
>>  		vma_shift = PAGE_SHIFT;
>>  	}
>>  
>> +	if (vma_shift == PUD_SHIFT &&
>> +	    !fault_supports_stage2_huge_mapping(memslot, hva, PUD_SIZE))
>> +		vma_shift = PMD_SHIFT;
>> +
>> +	if (vma_shift == PMD_SHIFT &&
>> +	    !fault_supports_stage2_huge_mapping(memslot, hva, PMD_SIZE)) {
>> +		force_pte = true;
>> +		vma_shift = PAGE_SHIFT;
>> +	}
>> +
>> +	vma_pagesize = 1UL << vma_shift;
>> +
>>  	/*
>>  	 * The stage2 has a minimum of 2 level table (For arm64 see
>>  	 * kvm_arm_setup_stage2()). Hence, we are guaranteed that we can
>> @@ -1889,7 +1898,7 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>>  	 */
>>  	if (vma_pagesize == PMD_SIZE ||
>>  	    (vma_pagesize == PUD_SIZE && kvm_stage2_has_pmd(kvm)))
>> -		gfn = (fault_ipa & huge_page_mask(hstate_vma(vma))) >> PAGE_SHIFT;
>> +		gfn = (fault_ipa & ~(vma_pagesize - 1)) >> PAGE_SHIFT;
>>  	mmap_read_unlock(current->mm);
>>  
>>  	/* We need minimum second+third level pages */
> Although this looks like a sensible change, I'm a reluctant to take it
> at this stage, given that we already have a bunch of patches from Will
> to change the way we deal with PTs.
>
> Could you look into how this could fit into the new code instead?

Sure, that sounds very sensible. I'm in the process of reviewing Will's series,
and after I'm done I'll rebase this on top of his patches and send it as v2. Does
that sound ok to you? Or do you want me to base this patch on one of your branches?

Thanks,
Alex