Re: [PATCH v2 2/2] arm64: tlb: Use the TLBI RANGE feature in arm64

Zhenyu Ye
2020-07-11T06:51:04Z

Hi Catalin,

On 2020/7/11 2:31, Catalin Marinas wrote:
> On Fri, Jul 10, 2020 at 05:44:20PM +0800, Zhenyu Ye wrote:
>> - if ((end - start) >= (MAX_TLBI_OPS * stride)) {
>> + if ((!cpus_have_const_cap(ARM64_HAS_TLBI_RANGE) &&
>> + (end - start) >= (MAX_TLBI_OPS * stride)) ||
>> + pages >= MAX_TLBI_RANGE_PAGES) {
>> flush_tlb_mm(vma->vm_mm);
>> return;
>> }
>
> I think we can use strictly greater here rather than greater or equal.
> MAX_TLBI_RANGE_PAGES can be encoded as num 31, scale 3.

Sorry, we can't.
For a boundary value (such as 2^6), we have two way to express it
in TLBI RANGE operations:
1. scale = 0, num = 31.
2. scale = 1, num = 0.

I used the second way in following implementation. However, for the
MAX_TLBI_RANGE_PAGES, we can only use scale = 3, num = 31.
So if use strictly greater here, ERROR will happen when range pages
equal to MAX_TLBI_RANGE_PAGES.

There are two ways to avoid this bug:
1. Just keep 'greater or equal' here. The ARM64 specification does
not specify how we flush tlb entries in this case, flush_tlb_mm()
is also a good choice for such a wide range of pages.
2. Add check in the loop, just like: (this may cause the codes a bit ugly)

num = __TLBI_RANGE_NUM(pages, scale) - 1;

/* scale = 4, num = 0 is equal to scale = 3, num = 31. */
if (scale == 4 && num == 0) {
scale = 3;
num = 31;
}

if (num >= 0) {
...

Which one do you prefer and how do you want to fix this error? Just
a fix patch again?

>
>>
>> - /* Convert the stride into units of 4k */
>> - stride >>= 12;
>> + dsb(ishst);
>>
>> - start = __TLBI_VADDR(start, asid);
>> - end = __TLBI_VADDR(end, asid);
>> + /*
>> + * When cpu does not support TLBI RANGE feature, we flush the tlb
>> + * entries one by one at the granularity of 'stride'.
>> + * When cpu supports the TLBI RANGE feature, then:
>> + * 1. If pages is odd, flush the first page through non-RANGE
>> + * instruction;
>> + * 2. For remaining pages: The minimum range granularity is decided
>> + * by 'scale', so we can not flush all pages by one instruction
>> + * in some cases.
>> + * Here, we start from scale = 0, flush corresponding pages
>> + * (from 2^(5*scale + 1) to 2^(5*(scale + 1) + 1)), and increase
>> + * it until no pages left.
>> + */
>> + while (pages > 0) {
>
> I did some simple checks on ((end - start) % stride) and never
> triggered. I had a slight worry that pages could become negative (and
> we'd loop forever since it's unsigned long) for some mismatched stride
> and flush size. It doesn't seem like.
>

The start and end are round_down/up in the function:

start = round_down(start, stride);
end = round_up(end, stride);

So the flush size and stride will never mismatch.

Thanks,
Zhenyu