Re: [RFC PATCH v2 2/2] riscv: mm: use svinval instructions instead of sfence.vma

[Mayuresh Chitale <mchitale@ventanamicro.com>]

On Fri, Dec 9, 2022 at 12:33 AM Palmer Dabbelt <palmer@dabbelt.com> wrote:
>
> On Sun, 24 Apr 2022 02:02:16 PDT (-0700), mchitale@ventanamicro.com wrote:
> > When svinval is supported the local_flush_tlb_page*
> > functions would prefer to use the following sequence
> > to optimize the tlb flushes instead of a simple sfence.vma:
> >
> > sfence.w.inval
> > svinval.vma
> >   .
> >   .
> > svinval.vma
> > sfence.inval.ir
> >
> > The maximum number of consecutive svinval.vma instructions
> > that can be executed in local_flush_tlb_page* functions is
> > limited to PTRS_PER_PTE. This is required to avoid soft
> > lockups and the approach is similar to that used in arm64.
> >
> > Signed-off-by: Mayuresh Chitale <mchitale@ventanamicro.com>
> > ---
> >  arch/riscv/include/asm/tlbflush.h |  12 ++++
> >  arch/riscv/kernel/setup.c         |   1 +
> >  arch/riscv/mm/tlbflush.c          | 116 ++++++++++++++++++++++++++++--
> >  3 files changed, 123 insertions(+), 6 deletions(-)
> >
> > diff --git a/arch/riscv/include/asm/tlbflush.h b/arch/riscv/include/asm/tlbflush.h
> > index 801019381dea..b535467c99f0 100644
> > --- a/arch/riscv/include/asm/tlbflush.h
> > +++ b/arch/riscv/include/asm/tlbflush.h
> > @@ -22,6 +22,18 @@ static inline void local_flush_tlb_page(unsigned long addr)
> >  {
> >       ALT_FLUSH_TLB_PAGE(__asm__ __volatile__ ("sfence.vma %0" : : "r" (addr) : "memory"));
> >  }
> > +
> > +void riscv_tlbflush_init(void);
> > +void __riscv_sfence_w_inval(void);
> > +void __riscv_sfence_inval_ir(void);
> > +void __riscv_sinval_vma(unsigned long addr);
> > +void __riscv_sinval_vma_asid(unsigned long addr, unsigned long asid);
> > +
> > +/* Check if we can use sinval for tlb flush */
> > +DECLARE_STATIC_KEY_FALSE(riscv_flush_tlb_svinval);
> > +#define riscv_use_flush_tlb_svinval() \
> > +     static_branch_unlikely(&riscv_flush_tlb_svinval)
> > +
> >  #else /* CONFIG_MMU */
> >  #define local_flush_tlb_all()                        do { } while (0)
> >  #define local_flush_tlb_page(addr)           do { } while (0)
> > diff --git a/arch/riscv/kernel/setup.c b/arch/riscv/kernel/setup.c
> > index 834eb652a7b9..13de04259de9 100644
> > --- a/arch/riscv/kernel/setup.c
> > +++ b/arch/riscv/kernel/setup.c
> > @@ -295,6 +295,7 @@ void __init setup_arch(char **cmdline_p)
> >  #endif
> >
> >       riscv_fill_hwcap();
> > +     riscv_tlbflush_init();
> >  }
> >
> >  static int __init topology_init(void)
> > diff --git a/arch/riscv/mm/tlbflush.c b/arch/riscv/mm/tlbflush.c
> > index 27a7db8eb2c4..800953f9121e 100644
> > --- a/arch/riscv/mm/tlbflush.c
> > +++ b/arch/riscv/mm/tlbflush.c
> > @@ -1,11 +1,14 @@
> >  // SPDX-License-Identifier: GPL-2.0
> >
> > +#define pr_fmt(fmt) "riscv: " fmt
> >  #include <linux/mm.h>
> >  #include <linux/smp.h>
> >  #include <linux/sched.h>
> >  #include <asm/sbi.h>
> >  #include <asm/mmu_context.h>
> >
> > +static unsigned long tlb_flush_all_threshold __read_mostly = PTRS_PER_PTE;
> > +
> >  static inline void local_flush_tlb_all_asid(unsigned long asid)
> >  {
> >       __asm__ __volatile__ ("sfence.vma x0, %0"
> > @@ -23,22 +26,110 @@ static inline void local_flush_tlb_page_asid(unsigned long addr,
> >                       : "memory");
> >  }
> >
> > +static inline void riscv_sfence_inval_ir(void)
> > +{
> > +     /*
> > +      * SFENCE.INVAL.IR
> > +      * 0001100 00001 00000 000 00000 1110011
> > +      */
> > +     asm volatile (".word 0x18100073" ::: "memory");
> > +}
> > +
> > +static inline void riscv_sfence_w_inval(void)
> > +{
> > +     /*
> > +      * SFENCE.W.INVAL
> > +      * 0001100 00000 00000 000 00000 1110011
> > +      */
> > +     asm volatile (".word 0x18000073" ::: "memory");
> > +}
> > +
> > +static inline void riscv_sinval_vma_asid(unsigned long vma, unsigned long asid)
> > +{
> > +     /*
> > +      * rs1 = a0 (VMA)
> > +      * rs2 = a1 (asid)
> > +      * SINVAL.VMA a0, a1
> > +      * 0001011 01011 01010 000 00000 1110011
> > +      */
> > +     asm volatile ("srli a0, %0, 2\n"
> > +                     "add a1, %1, zero\n"
> > +                     ".word 0x16B50073\n"
> > +                     :: "r" (vma), "r" (asid)
> > +                     : "a0", "a1", "memory");
> > +}
> > +
> > +static inline void riscv_sinval_vma(unsigned long vma)
> > +{
> > +     /*
> > +      * rs1 = a0 (VMA)
> > +      * rs2 = 0
> > +      * SINVAL.VMA a0
> > +      * 0001011 00000 01010 000 00000 1110011
> > +      */
> > +     asm volatile ("srli a0, %0, 2\n"
> > +                     ".word 0x16050073\n"
> > +                     :: "r" (vma) : "a0", "memory");
> > +}
> > +
> >  static inline void local_flush_tlb_range(unsigned long start,
> >               unsigned long size, unsigned long stride)
> >  {
> > -     if (size <= stride)
> > -             local_flush_tlb_page(start);
> > -     else
> > +     if ((size / stride) <= tlb_flush_all_threshold) {
> > +             if (riscv_use_flush_tlb_svinval()) {
> > +                     riscv_sfence_w_inval();
> > +                     while (size) {
> > +                             riscv_sinval_vma(start);
> > +                             start += stride;
> > +                             if (size > stride)
> > +                                     size -= stride;
> > +                             else
> > +                                     size = 0;
> > +                     }
> > +                     riscv_sfence_inval_ir();
> > +             } else {
> > +                     while (size) {
> > +                             local_flush_tlb_page(start);
> > +                             start += stride;
> > +                             if (size > stride)
> > +                                     size -= stride;
> > +                             else
> > +                                     size = 0;
> > +                     }
> > +             }
> > +     } else {
> >               local_flush_tlb_all();
> > +     }
> >  }
> >
> >  static inline void local_flush_tlb_range_asid(unsigned long start,
> >               unsigned long size, unsigned long stride, unsigned long asid)
> >  {
> > -     if (size <= stride)
> > -             local_flush_tlb_page_asid(start, asid);
> > -     else
> > +     if ((size / stride) <= tlb_flush_all_threshold) {
> > +             if (riscv_use_flush_tlb_svinval()) {
> > +                     riscv_sfence_w_inval();
> > +                     while (size) {
> > +                             riscv_sinval_vma_asid(start, asid);
> > +                             start += stride;
> > +                             if (size > stride)
> > +                                     size -= stride;
> > +                             else
> > +                                     size = 0;
> > +                     }
> > +                     riscv_sfence_inval_ir();
> > +             } else {
> > +                     while (size) {
> > +                             local_flush_tlb_page_asid(start, asid);
> > +                             start += stride;
> > +                             if (size > stride)
> > +                                     size -= stride;
> > +                             else
> > +                                     size = 0;
> > +                     }
> > +             }
> > +     } else {
> >               local_flush_tlb_all_asid(asid);
> > +     }
>
> Sorry to dig up an old post, but svinval came up and I started looking
> at this again.
>
> There's some other issues with the code, but at a higher level it's not
> really clear this is a useful thing to do: splitting up sfence.vma makes
> sense, but if there's no operations between the sfence.w.inval and
> sinval.vma (or sinval.vma and sfence.inval.ir) then we're not giving the
> HW anything else to do concurrently with the flushes and thus I don't
> see how this would improve performance -- if anything we're just
> emitting more instructions to do the samething, which would be bad for
> performance.
Actually this approach is similar to ARM with addition of only two
instructions: sfence.w.inval and sfence.inval.ir and it is also
described in the non normative text in the priv spec ch.7.

>
> So I think if we're going to use these we really need to also split up
> the TLB flush operations, so we can give something for the HW to do
> currently with the flushing.  That's be a pretty big change to our TLB
> model, though: essentially we're going from two phases (write the page
> tables, then flush) to at least three phases (write the page tables,
> start the flush, finish the flush) or even four (if we want to insert
> work between both sfence.w.inval->sinval.vma and
> sinval.vma->sfence.inval.ir).
I think the extension is just to allow for a more efficient TLB
flushing operation in the hardware and no change in the software TLB
model is expected.
Besides, the extension would be disabled by default and only get
enabled on platforms that enable it in DT.

>
> Not sure if I'm just misunderstanding the performance characteristics,
> though.   Maybe I'm over-thinking things and sfence.vma is heavy-weight
> enough that implementations are flushing the pipeline for even the
> page-based flavors and thus sfence.vma-based loops are very slow?


>
> Do you have any benchmarks for the Svinval bits here (which are mixed in
> with some non-global flushing, that's a different optimization that
> needs to be split out)?
>
>
> >  }
> >
> >  static void __ipi_flush_tlb_all(void *info)
> > @@ -149,3 +240,16 @@ void flush_pmd_tlb_range(struct vm_area_struct *vma, unsigned long start,
> >       __flush_tlb_range(vma->vm_mm, start, end - start, PMD_SIZE);
> >  }
> >  #endif
> > +
> > +DEFINE_STATIC_KEY_FALSE(riscv_flush_tlb_svinval);
> > +EXPORT_SYMBOL_GPL(riscv_flush_tlb_svinval);
> > +
> > +void riscv_tlbflush_init(void)
> > +{
> > +     if (riscv_isa_extension_available(NULL, SVINVAL)) {
> > +             pr_info("Svinval extension supported\n");
> > +             static_branch_enable(&riscv_flush_tlb_svinval);
> > +     } else {
> > +             static_branch_disable(&riscv_flush_tlb_svinval);
> > +     }
> > +}

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