Re: [RFC PATCH 2/4] KVM: selftests: Align memory region addresses to 1M on s390x

[Thomas Huth <thuth@redhat.com>]

On 16/05/2019 13.30, David Hildenbrand wrote:
> On 16.05.19 13:12, Thomas Huth wrote:
>> On s390x, there is a constraint that memory regions have to be aligned
>> to 1M (or running the VM will fail). Introduce a new "alignment" variable
>> in the vm_userspace_mem_region_add() function which now can be used for
>> both, huge page and s390x alignment requirements.
>>
>> Signed-off-by: Thomas Huth <thuth@redhat.com>
>> ---
>>  tools/testing/selftests/kvm/lib/kvm_util.c | 21 +++++++++++++++++-----
>>  1 file changed, 16 insertions(+), 5 deletions(-)
>>
>> diff --git a/tools/testing/selftests/kvm/lib/kvm_util.c b/tools/testing/selftests/kvm/lib/kvm_util.c
>> index 8d63ccb93e10..64a0da6efe3d 100644
>> --- a/tools/testing/selftests/kvm/lib/kvm_util.c
>> +++ b/tools/testing/selftests/kvm/lib/kvm_util.c
>> @@ -559,6 +559,7 @@ void vm_userspace_mem_region_add(struct kvm_vm *vm,
>>  	unsigned long pmem_size = 0;
>>  	struct userspace_mem_region *region;
>>  	size_t huge_page_size = KVM_UTIL_PGS_PER_HUGEPG * vm->page_size;
>> +	size_t alignment;
>>  
>>  	TEST_ASSERT((guest_paddr % vm->page_size) == 0, "Guest physical "
>>  		"address not on a page boundary.\n"
>> @@ -608,9 +609,20 @@ void vm_userspace_mem_region_add(struct kvm_vm *vm,
>>  	TEST_ASSERT(region != NULL, "Insufficient Memory");
>>  	region->mmap_size = npages * vm->page_size;
>>  
>> -	/* Enough memory to align up to a huge page. */
>> +#ifdef __s390x__
>> +	/* On s390x, the host address must be aligned to 1M (due to PGSTEs) */
>> +	alignment = 0x100000;
> 
> This corresponds to huge_page_size, maybe you can exploit this fact here.
> 
> Something like
> 
> alignment = 1;
> 
> /* On s390x, the host address must always be aligned to the THP size */
> #ifndef __s390x__
> if (src_type == VM_MEM_SRC_ANONYMOUS_THP)
> #endif
> 	alignment = huge_page_size;
> 
> Maybe in a nicer fashion. Not sure.

Hmm, but if I've got your explanation on IRC right, it's rather a
coincidence that the huge page size matches the alignment requirements
for KVM memslots, isn't it? So I think the code would look rather
confusing if I'd try to shorten it this way...?

 Thomas