On Mon, Feb 17, 2020 at 1:17 PM Robin Murphy <robin.mur...@arm.com> wrote:
>
> On 13/02/2020 9:49 pm, Rob Herring wrote:
> > On Thu, Jan 30, 2020 at 11:34 AM Robin Murphy <robin.mur...@arm.com> wrote:
> >>
> >> On 30/01/2020 3:06 pm, Auger Eric wrote:
> >>> Hi Rob,
> >>> On 1/17/20 10:16 PM, Rob Herring wrote:
> >>>> Arm SMMUv3.2 adds support for TLB range invalidate operations.
> >>>> Support for range invalidate is determined by the RIL bit in the IDR3
> >>>> register.
> >>>>
> >>>> The range invalidate is in units of the leaf page size and operates on
> >>>> 1-32 chunks of a power of 2 multiple pages. First, we determine from the
> >>>> size what power of 2 multiple we can use. Then we calculate how many
> >>>> chunks (1-31) of the power of 2 size for the range on the iteration. On
> >>>> each iteration, we move up in size by at least 5 bits.
> >>>>
> >>>> Cc: Eric Auger <eric.au...@redhat.com>
> >>>> Cc: Jean-Philippe Brucker <jean-phili...@linaro.org>
> >>>> Cc: Will Deacon <w...@kernel.org>
> >>>> Cc: Robin Murphy <robin.mur...@arm.com>
> >>>> Cc: Joerg Roedel <j...@8bytes.org>
> >>>> Signed-off-by: Rob Herring <r...@kernel.org>
> >
> >
> >>>> @@ -2003,7 +2024,7 @@ static void arm_smmu_tlb_inv_range(unsigned long
> >>>> iova, size_t size,
> >>>> {
> >>>> u64 cmds[CMDQ_BATCH_ENTRIES * CMDQ_ENT_DWORDS];
> >>>> struct arm_smmu_device *smmu = smmu_domain->smmu;
> >>>> - unsigned long start = iova, end = iova + size;
> >>>> + unsigned long start = iova, end = iova + size, num_pages = 0, tg =
> >>>> 0;
> >>>> int i = 0;
> >>>> struct arm_smmu_cmdq_ent cmd = {
> >>>> .tlbi = {
> >>>> @@ -2022,12 +2043,50 @@ static void arm_smmu_tlb_inv_range(unsigned long
> >>>> iova, size_t size,
> >>>> cmd.tlbi.vmid = smmu_domain->s2_cfg.vmid;
> >>>> }
> >>>>
> >>>> + if (smmu->features & ARM_SMMU_FEAT_RANGE_INV) {
> >>>> + /* Get the leaf page size */
> >>>> + tg = __ffs(smmu_domain->domain.pgsize_bitmap);
> >>>> +
> >>>> + /* Convert page size of 12,14,16 (log2) to 1,2,3 */
> >>>> + cmd.tlbi.tg = ((tg - ilog2(SZ_4K)) / 2) + 1;
> >>
> >> Given the comment, I think "(tg - 10) / 2" would suffice ;)
> >
> > Well, duh...
> >
> >>
> >>>> +
> >>>> + /* Determine what level the granule is at */
> >>>> + cmd.tlbi.ttl = 4 - ((ilog2(granule) - 3) / (tg - 3));
> >>
> >> Is it possible to rephrase that with logs and shifts to avoid the division?
> >
> > Well, with a loop is the only other way I came up with:
> >
> > bpl = tg - 3;
> > ttl = 3;
> > mask = BIT(bpl) - 1;
> > while ((granule & (mask << ((4 - ttl) * bpl + 3))) == 0)
> > ttl--;
> >
> > Doesn't seem like much improvement to me given we have h/w divide...
>
> Sure, it doesn't take too many extra instructions to start matching
> typical IDIV latency, so there's no point being silly if there really
> isn't a clean alternative.
>
> Some quick scribbling suggests "4 - ilog2(granule) / 10" might actually
> be close enough, but perhaps that's a bit too cheeky.
How does divide by 10 save a divide?
> >>>> + num_pages = size / (1UL << tg);
> >>
> >> Similarly, in my experience GCC has always seemed too cautious to elide
> >> non-constant division even in a seemingly-obvious case like this, so
> >> explicit "size >> tg" might be preferable.
> >
> > My experience has been gcc is smart enough. I checked and there's only
> > one divide and it is the prior one. But I'll change it anyways.
>
> Now that I think about it, the case that frustrated me may have had the
> shift and divide in separate statements - that's probably a lot harder
> to analyse than a single expression. Either way, the simple right shift
> is easier to read IMO.
>
> >>>> + }
> >>>> +
> >>>> while (iova < end) {
> >>>> if (i == CMDQ_BATCH_ENTRIES) {
> >>>> arm_smmu_cmdq_issue_cmdlist(smmu, cmds, i, false);
> >>>> i = 0;
> >>>> }
> >>>>
> >>>> + if (smmu->features & ARM_SMMU_FEAT_RANGE_INV) {
> >>>> + /*
> >>>> + * On each iteration of the loop, the range is 5
> >>>> bits
> >>>> + * worth of the aligned size remaining.
> >>>> + * The range in pages is:
> >>>> + *
> >>>> + * range = (num_pages & (0x1f << __ffs(num_pages)))
> >>>> + */
> >>>> + unsigned long scale, num;
> >>>> +
> >>>> + /* Determine the power of 2 multiple number of
> >>>> pages */
> >>>> + scale = __ffs(num_pages);
> >>>> + cmd.tlbi.scale = scale;
> >>>> +
> >>>> + /* Determine how many chunks of 2^scale size we
> >>>> have */
> >>>> + num = (num_pages >> scale) &
> >>>> CMDQ_TLBI_RANGE_NUM_MAX;
> >>>> + cmd.tlbi.num = num - 1;
> >>>> +
> >>>> + /* range is num * 2^scale * pgsize */
> >>>> + granule = num << (scale + tg);
> >>>> +
> >>>> + /* Clear out the lower order bits for the next
> >>>> iteration */
> >>>> + num_pages -= num << scale;
> >>> Regarding the 2 options given in
> >>> https://lore.kernel.org/linux-arm-kernel/CAL_JsqKABoE+0crGwyZdNogNgEoG=moopf6deqgh6s73c0u...@mail.gmail.com/raw,
> >>>
> >>> I understand you implemented 2) but I still do not understand why you
> >>> preferred that one against 1).
> >>>
> >>> In your case of 1023*4k pages this will invalidate by 31 32*2^0*4K +
> >>> 31*2^0*4K pages
> >>> whereas you could achieve that with 10 invalidations with the 1st algo.
> >>> I did not get the case where it is more efficient. Please can you detail.
> >>
> >> I guess essentially we want to solve for two variables to get a range as
> >> close to size as possible. There might be a more elegant numerical
> >> method, but for the numbers involved brute force is probably good enough
> >> for the real world. 5 minutes alone with the architecture spec and a
> >> blank editor begat this pseudo-implementation:
> >>
> >> size_t npages = size >> pgshift;
> >> while (npages) {
> >> unsigned long range;
> >> unsigned int delta, best = UINT_MAX;
> >> int num, scale = min(31, __ffs(npages));
> >>
> >> while (scale) {
> >> num = min(32, npages >> scale);
> >> if (num == 32)
> >> break;
> >>
> >> delta = npages & ((1 << scale) - 1);
> >> if (!delta || delta > best)
> >> break;
> >>
> >> best = delta;
> >> scale--;
> >> }
> >>
> >> //invalidate
> >>
> >> range = num << scale;
> >> npages -= range;
> >> iova += (range) << pgshift;
> >> }
> >>
> >> Modulo any obvious thinkos, what do you reckon?
> >
> > I don't think this is an improvement. See my other reply.
>
> Indeed, I hadn't quite got my head round your algorithm at first, so
> deriving this was as much to help me get a better feel for the problem
> as anything. Now I see that "minimise the remainder" really boils down
> to "remove up to 5 high-order bits each time", which in turn is
> essentially the same thing just done in the other direction (and in a
> slightly more cumbersome manner). Now that I do get it, your algorithm
> is in fact really neat, sorry for doubting :)
>
> FWIW it might be a little more efficient to maintain scale outside the
> loop, such that num_pages can simply be shifted right to lose the
> low-order bits each iteration, but other than that I think it's pretty
> much as good as it can get.
I think I understand what you're getting at. We'd need to have the
total running 'scale' and the incremental 'scale' for each loop
iteration. We could do that by using cmd.tlbi.scale to keep the
running total. So something like this (untested)?:
if (smmu->features & ARM_SMMU_FEAT_RANGE_INV) {
unsigned long scale, num;
/* Determine the power of 2 multiple number of pages */
scale = __ffs(num_pages);
cmd.tlbi.scale += scale;
/* Determine how many chunks of 2^scale size we have */
num = (num_pages >> scale) & CMDQ_TLBI_RANGE_NUM_MAX;
cmd.tlbi.num = num - 1;
/* range is num * 2^scale * pgsize */
granule = num << (cmd.tlbi.scale + tg);
/* Clear out the lower order bits for the next iteration */
num_pages >>= 5 + scale;
}
That's actually worse by 1 add and probably a memory (re)load of
cmd.tlbi.scale AFAICT.
Rob
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