Re: [RFC PATCH v2] arm64: cpufeatures: add support for tlbi range instructions

Olof Johansson
2019-11-11T16:33:08Z

On Mon, Nov 11, 2019 at 5:24 AM Zhenyu Ye <yezhenyu2@huawei.com> wrote:
>
> ARMv8.4-TLBI provides TLBI invalidation instruction that apply to a
> range of input addresses. This patch adds support for this feature.
> This is the second version of the patch.
>
> I traced the __flush_tlb_range() for a minute and get some statistical
> data as below:
>
> PAGENUM COUNT
> 1 34944
> 2 5683
> 3 1343
> 4 7857
> 5 838
> 9 339
> 16 933
> 19 427
> 20 5821
> 23 279
> 41 338
> 141 279
> 512 428
> 1668 120
> 2038 100
>
> Those data are based on kernel-5.4.0, where PAGENUM = end - start, COUNT
> shows number of calls to the __flush_tlb_range() in a minute. There only
> shows the data which COUNT >= 100. The kernel is started normally, and
> transparent hugepage is opened. As we can see, though most user TLBI
> ranges were 1 pages long, the num of long-range can not be ignored.
>
> The new feature of TLB range can improve lots of performance compared to
> the current implementation. As an example, flush 512 ranges needs only 1
> instruction as opposed to 512 instructions using current implementation.

But there's no indication whether this performs better or not in reality.

A perf report indicating, for example, cycles spent in TLBI on the two
versions would be a lot more valuable.

> And for a new hardware feature, support is better than not.

This is blatantly untrue. Complexity is added, and if there's no
evidence of benefit of said complexity, it is not something we want to
add.

> Signed-off-by: Zhenyu Ye <yezhenyu2@huawei.com>
> ---
> ChangeLog v1 -> v2:
> - Change the main implementation of this feature.
> - Add some comments.
>
> ---
> arch/arm64/include/asm/cpucaps.h | 3 +-
> arch/arm64/include/asm/sysreg.h | 4 ++
> arch/arm64/include/asm/tlbflush.h | 99 ++++++++++++++++++++++++++++++-
> arch/arm64/kernel/cpufeature.c | 10 ++++
> 4 files changed, 112 insertions(+), 4 deletions(-)
>
> diff --git a/arch/arm64/include/asm/cpucaps.h b/arch/arm64/include/asm/cpucaps.h
> index ac1dbca3d0cd..5b5230060e5b 100644
> --- a/arch/arm64/include/asm/cpucaps.h
> +++ b/arch/arm64/include/asm/cpucaps.h
> @@ -54,7 +54,8 @@
> #define ARM64_WORKAROUND_1463225 44
> #define ARM64_WORKAROUND_CAVIUM_TX2_219_TVM 45
> #define ARM64_WORKAROUND_CAVIUM_TX2_219_PRFM 46
> +#define ARM64_HAS_TLBI_RANGE 47
>
> -#define ARM64_NCAPS 47
> +#define ARM64_NCAPS 48
>
> #endif /* __ASM_CPUCAPS_H */
> diff --git a/arch/arm64/include/asm/sysreg.h b/arch/arm64/include/asm/sysreg.h
> index 6e919fafb43d..a6abbf2b067d 100644
> --- a/arch/arm64/include/asm/sysreg.h
> +++ b/arch/arm64/include/asm/sysreg.h
> @@ -539,6 +539,7 @@
> ENDIAN_SET_EL1 | SCTLR_EL1_UCI | SCTLR_EL1_RES1)
>
> /* id_aa64isar0 */
> +#define ID_AA64ISAR0_TLB_SHIFT 56
> #define ID_AA64ISAR0_TS_SHIFT 52
> #define ID_AA64ISAR0_FHM_SHIFT 48
> #define ID_AA64ISAR0_DP_SHIFT 44
> @@ -552,6 +553,9 @@
> #define ID_AA64ISAR0_SHA1_SHIFT 8
> #define ID_AA64ISAR0_AES_SHIFT 4
>
> +#define ID_AA64ISAR0_TLB_NI 0x0
> +#define ID_AA64ISAR0_TLB_RANGE 0x2
> +
> /* id_aa64isar1 */
> #define ID_AA64ISAR1_SB_SHIFT 36
> #define ID_AA64ISAR1_FRINTTS_SHIFT 32
> diff --git a/arch/arm64/include/asm/tlbflush.h b/arch/arm64/include/asm/tlbflush.h
> index bc3949064725..f49bed7ecb68 100644
> --- a/arch/arm64/include/asm/tlbflush.h
> +++ b/arch/arm64/include/asm/tlbflush.h
> @@ -59,6 +59,33 @@
> __ta; \
> })
>
> +/*
> + * This macro creates a properly formatted VA operand for the TLBI RANGE.
> + * The value bit assignments are:
> + *
> + * +----------+------+-------+-------+-------+----------------------+
> + * | ASID | TG | SCALE | NUM | TTL | BADDR |
> + * +-----------------+-------+-------+-------+----------------------+
> + * |63 48|47 46|45 44|43 39|38 37|36 0|
> + *
> + * The address range is determined by below formula:
> + * [BADDR, BADDR + (NUM + 1) * 2^(5*SCALE + 1) * PAGESIZE)
> + *
> + */
> +#define __TLBI_VADDR_RANGE(addr, asid, tg, scale, num, ttl) \
> + ({ \
> + unsigned long __ta = (addr) >> PAGE_SHIFT; \
> + __ta |= (unsigned long)(ttl) << 37; \
> + __ta |= (unsigned long)(num) << 39; \
> + __ta |= (unsigned long)(scale) << 44; \
> + __ta |= (unsigned long)(tg) << 46; \
> + __ta |= (unsigned long)(asid) << 48; \
> + __ta; \
> + })
> +
> +#define TLB_RANGE_MASK_SHIFT 5
> +#define TLB_RANGE_MASK GENMASK_ULL(TLB_RANGE_MASK_SHIFT, 0)
> +
> /*
> * TLB Invalidation
> * ================
> @@ -177,9 +204,9 @@ static inline void flush_tlb_page(struct vm_area_struct *vma,
> */
> #define MAX_TLBI_OPS PTRS_PER_PTE
>
> -static inline void __flush_tlb_range(struct vm_area_struct *vma,
> - unsigned long start, unsigned long end,
> - unsigned long stride, bool last_level)
> +static inline void __flush_tlb_range_old(struct vm_area_struct *vma,
> + unsigned long start, unsigned long end,
> + unsigned long stride, bool last_level)

"old" and "new" are not very descriptive.

> {
> unsigned long asid = ASID(vma->vm_mm);
> unsigned long addr;
> @@ -211,6 +238,72 @@ static inline void __flush_tlb_range(struct vm_area_struct *vma,
> dsb(ish);
> }
>
> +static inline void __flush_tlb_range_new(struct vm_area_struct *vma,
> + unsigned long start, unsigned long end,
> + unsigned long stride, bool last_level)
> +{
> + int num = 0;
> + int scale = 0;
> + int ttl = 0;
> + int tg = (PAGE_SHIFT - 12) / 2 + 1;

This is a constant, and shouldn't need to be a variable. You can push
it down to the addr generator macro.

> + unsigned long asid = ASID(vma->vm_mm);
> + unsigned long addr = 0;
> + unsigned long offset = (end - start) >> PAGE_SHIFT;

"offset" confused me a lot here -- I think this variable really
describes number of pages to flush?

And, if so, you're probably off-by-one here: you need to round up
partial pages. As a matter of fact, you probably need to deal with
partial pages at both beginning and end.

> + if (offset > (1UL << 21)) {
> + flush_tlb_mm(vma->vm_mm);
> + return;
> + }

There's a comment that this limitation on iterative flushes is
arbitrary, and selected to not trigger soft lockups. At the very
least, you need a similar comment here as to why this code is needed.

> + dsb(ishst);
> +
> + /*
> + * The minimum size of TLB RANGE is 2 PAGE;
> + * Use normal TLB instruction to handle odd PAGEs
> + */
> + if (offset % 2 == 1) {
> + addr = __TLBI_VADDR(start, asid);
> + if (last_level) {
> + __tlbi(vale1is, addr);
> + __tlbi_user(vale1is, addr);
> + } else {
> + __tlbi(vae1is, addr);
> + __tlbi_user(vae1is, addr);
> + }
> + start += 1 << PAGE_SHIFT;
> + offset -= 1;
> + }
> +
> + while (offset > 0) {
> + num = (offset & TLB_RANGE_MASK) - 1;
> + if (num >= 0) {
> + addr = __TLBI_VADDR_RANGE(start, asid, tg,
> + scale, num, ttl);
> + if (last_level) {
> + __tlbi(rvale1is, addr);
> + __tlbi_user(rvale1is, addr);
> + } else {
> + __tlbi(rvae1is, addr);
> + __tlbi_user(rvae1is, addr);
> + }
> + start += (num + 1) << (5 * scale + 1) << PAGE_SHIFT;
> + }
> + scale++;

This is an odd way of doing the loop, by looping over the base and
linearly increasing the exponent.

Wouldn't it be easier to start with as high 'num' as possible as long
as the range ("offset" in your code) is larger than 2^5, and then do a
few iterations at the end for the smaller ranges?

> + offset >>= TLB_RANGE_MASK_SHIFT;
> + }
> + dsb(ish);
> +}

The inner pieces of this loop, the special case at the beginning, and
the old implementation are all the same.

The main difference between now and before are:

1) How much you step forward on each iteration
2) How you calculate the address argument

Would it be better to just refactor the old code? You can calculate
the ranges the same way but just loop over them for non-8.4-TLBI
platforms.

No matter what, we really want some benchmarks and numbers to motivate
these changes. TLB operations tend to get on critical paths where
single cycles matter.

> +static inline void __flush_tlb_range(struct vm_area_struct *vma,
> + unsigned long start, unsigned long end,
> + unsigned long stride, bool last_level)
> +{
> + if (cpus_have_const_cap(ARM64_HAS_TLBI_RANGE))
> + __flush_tlb_range_new(vma, start, end, stride, last_level);
> + else
> + __flush_tlb_range_old(vma, start, end, stride, last_level);
> +}
> +
> static inline void flush_tlb_range(struct vm_area_struct *vma,
> unsigned long start, unsigned long end)
> {
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index 80f459ad0190..bdefd8a34729 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -1566,6 +1566,16 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
> .min_field_value = 1,
> },
> #endif
> + {
> + .desc = "TLB range maintenance instruction",
> + .capability = ARM64_HAS_TLBI_RANGE,
> + .type = ARM64_CPUCAP_SYSTEM_FEATURE,
> + .matches = has_cpuid_feature,
> + .sys_reg = SYS_ID_AA64ISAR0_EL1,
> + .field_pos = ID_AA64ISAR0_TLB_SHIFT,
> + .sign = FTR_UNSIGNED,
> + .min_field_value = ID_AA64ISAR0_TLB_RANGE,
> + },
> {},
> };
>
> --
> 2.19.1
>
>