Date:   Tue, 5 Oct 2021 09:57:06 -0400
From:   Darius Rad <darius@...espec.com>
To:     Greentime Hu <greentime.hu@...ive.com>
Cc:     linux-riscv <linux-riscv@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Albert Ou <aou@...s.berkeley.edu>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Vincent Chen <vincent.chen@...ive.com>
Subject: Re: [RFC PATCH v8 09/21] riscv: Add task switch support for vector

On Mon, Oct 04, 2021 at 08:36:30PM +0800, Greentime Hu wrote:
> Darius Rad <darius@...espec.com> 於 2021年9月29日 週三 下午9:28寫道：
> >
> > On Tue, Sep 28, 2021 at 10:56:52PM +0800, Greentime Hu wrote:
> > > Darius Rad <darius@...espec.com> 於 2021年9月13日 週一 下午8:21寫道：
> > > >
> > > > On 9/8/21 1:45 PM, Greentime Hu wrote:
> > > > > This patch adds task switch support for vector. It supports partial lazy
> > > > > save and restore mechanism. It also supports all lengths of vlen.
> > > > >
> > > > > [guoren@...ux.alibaba.com: First available porting to support vector
> > > > > context switching]
> > > > > [nick.knight@...ive.com: Rewrite vector.S to support dynamic vlen, xlen and
> > > > > code refine]
> > > > > [vincent.chen@...ive.co: Fix the might_sleep issue in vstate_save,
> > > > > vstate_restore]
> > > > > Co-developed-by: Nick Knight <nick.knight@...ive.com>
> > > > > Signed-off-by: Nick Knight <nick.knight@...ive.com>
> > > > > Co-developed-by: Guo Ren <guoren@...ux.alibaba.com>
> > > > > Signed-off-by: Guo Ren <guoren@...ux.alibaba.com>
> > > > > Co-developed-by: Vincent Chen <vincent.chen@...ive.com>
> > > > > Signed-off-by: Vincent Chen <vincent.chen@...ive.com>
> > > > > Signed-off-by: Greentime Hu <greentime.hu@...ive.com>
> > > > > ---
> > > > >   arch/riscv/include/asm/switch_to.h | 66 +++++++++++++++++++++++
> > > > >   arch/riscv/kernel/Makefile         |  1 +
> > > > >   arch/riscv/kernel/process.c        | 38 ++++++++++++++
> > > > >   arch/riscv/kernel/vector.S         | 84 ++++++++++++++++++++++++++++++
> > > > >   4 files changed, 189 insertions(+)
> > > > >   create mode 100644 arch/riscv/kernel/vector.S
> > > > >
> > > > > diff --git a/arch/riscv/include/asm/switch_to.h b/arch/riscv/include/asm/switch_to.h
> > > > > index ec83770b3d98..de0573dad78f 100644
> > > > > --- a/arch/riscv/include/asm/switch_to.h
> > > > > +++ b/arch/riscv/include/asm/switch_to.h
> > > > > @@ -7,10 +7,12 @@
> > > > >   #define _ASM_RISCV_SWITCH_TO_H
> > > > >
> > > > >   #include <linux/jump_label.h>
> > > > > +#include <linux/slab.h>
> > > > >   #include <linux/sched/task_stack.h>
> > > > >   #include <asm/processor.h>
> > > > >   #include <asm/ptrace.h>
> > > > >   #include <asm/csr.h>
> > > > > +#include <asm/asm-offsets.h>
> > > > >
> > > > >   #ifdef CONFIG_FPU
> > > > >   extern void __fstate_save(struct task_struct *save_to);
> > > > > @@ -68,6 +70,68 @@ static __always_inline bool has_fpu(void) { return false; }
> > > > >   #define __switch_to_fpu(__prev, __next) do { } while (0)
> > > > >   #endif
> > > > >
> > > > > +#ifdef CONFIG_VECTOR
> > > > > +extern bool has_vector;
> > > > > +extern unsigned long riscv_vsize;
> > > > > +extern void __vstate_save(struct __riscv_v_state *save_to, void *datap);
> > > > > +extern void __vstate_restore(struct __riscv_v_state *restore_from, void *datap);
> > > > > +
> > > > > +static inline void __vstate_clean(struct pt_regs *regs)
> > > > > +{
> > > > > +     regs->status = (regs->status & ~(SR_VS)) | SR_VS_CLEAN;
> > > > > +}
> > > > > +
> > > > > +static inline void vstate_off(struct task_struct *task,
> > > > > +                           struct pt_regs *regs)
> > > > > +{
> > > > > +     regs->status = (regs->status & ~SR_VS) | SR_VS_OFF;
> > > > > +}
> > > > > +
> > > > > +static inline void vstate_save(struct task_struct *task,
> > > > > +                            struct pt_regs *regs)
> > > > > +{
> > > > > +     if ((regs->status & SR_VS) == SR_VS_DIRTY) {
> > > > > +             struct __riscv_v_state *vstate = &(task->thread.vstate);
> > > > > +
> > > > > +             __vstate_save(vstate, vstate->datap);
> > > > > +             __vstate_clean(regs);
> > > > > +     }
> > > > > +}
> > > > > +
> > > > > +static inline void vstate_restore(struct task_struct *task,
> > > > > +                               struct pt_regs *regs)
> > > > > +{
> > > > > +     if ((regs->status & SR_VS) != SR_VS_OFF) {
> > > > > +             struct __riscv_v_state *vstate = &(task->thread.vstate);
> > > > > +
> > > > > +             /* Allocate space for vector registers. */
> > > > > +             if (!vstate->datap) {
> > > > > +                     vstate->datap = kzalloc(riscv_vsize, GFP_ATOMIC);
> > > > > +                     vstate->size = riscv_vsize;
> > > > > +             }
> > > > > +             __vstate_restore(vstate, vstate->datap);
> > > > > +             __vstate_clean(regs);
> > > > > +     }
> > > > > +}
> > > > > +
> > > > > +static inline void __switch_to_vector(struct task_struct *prev,
> > > > > +                                struct task_struct *next)
> > > > > +{
> > > > > +     struct pt_regs *regs;
> > > > > +
> > > > > +     regs = task_pt_regs(prev);
> > > > > +     if (unlikely(regs->status & SR_SD))
> > > > > +             vstate_save(prev, regs);
> > > > > +     vstate_restore(next, task_pt_regs(next));
> > > > > +}
> > > > > +
> > > > > +#else
> > > > > +#define has_vector false
> > > > > +#define vstate_save(task, regs) do { } while (0)
> > > > > +#define vstate_restore(task, regs) do { } while (0)
> > > > > +#define __switch_to_vector(__prev, __next) do { } while (0)
> > > > > +#endif
> > > > > +
> > > > >   extern struct task_struct *__switch_to(struct task_struct *,
> > > > >                                      struct task_struct *);
> > > > >
> > > > > @@ -77,6 +141,8 @@ do {                                                       \
> > > > >       struct task_struct *__next = (next);            \
> > > > >       if (has_fpu())                                  \
> > > > >               __switch_to_fpu(__prev, __next);        \
> > > > > +     if (has_vector)                                 \
> > > > > +             __switch_to_vector(__prev, __next);     \
> > > > >       ((last) = __switch_to(__prev, __next));         \
> > > > >   } while (0)
> > > > >
> > > > > diff --git a/arch/riscv/kernel/Makefile b/arch/riscv/kernel/Makefile
> > > > > index 3397ddac1a30..344078080839 100644
> > > > > --- a/arch/riscv/kernel/Makefile
> > > > > +++ b/arch/riscv/kernel/Makefile
> > > > > @@ -40,6 +40,7 @@ obj-$(CONFIG_MMU) += vdso.o vdso/
> > > > >
> > > > >   obj-$(CONFIG_RISCV_M_MODE)  += traps_misaligned.o
> > > > >   obj-$(CONFIG_FPU)           += fpu.o
> > > > > +obj-$(CONFIG_VECTOR)         += vector.o
> > > > >   obj-$(CONFIG_SMP)           += smpboot.o
> > > > >   obj-$(CONFIG_SMP)           += smp.o
> > > > >   obj-$(CONFIG_SMP)           += cpu_ops.o
> > > > > diff --git a/arch/riscv/kernel/process.c b/arch/riscv/kernel/process.c
> > > > > index 03ac3aa611f5..0b86e9e531c9 100644
> > > > > --- a/arch/riscv/kernel/process.c
> > > > > +++ b/arch/riscv/kernel/process.c
> > > > > @@ -95,6 +95,16 @@ void start_thread(struct pt_regs *regs, unsigned long pc,
> > > > >                */
> > > > >               fstate_restore(current, regs);
> > > > >       }
> > > > > +
> > > > > +     if (has_vector) {
> > > > > +             regs->status |= SR_VS_INITIAL;
> > > > > +             /*
> > > > > +              * Restore the initial value to the vector register
> > > > > +              * before starting the user program.
> > > > > +              */
> > > > > +             vstate_restore(current, regs);
> > > > > +     }
> > > > > +
> > > >
> > > > So this will unconditionally enable vector instructions, and allocate
> > > > memory for vector state, for all processes, regardless of whether vector
> > > > instructions are used?
> > > >
> > >
> > > Hi Darius,
> > >
> > > Yes, it will enable vector if has_vector() is true. The reason that we
> > > choose to enable and allocate memory for user space program is because
> > > we also implement some common functions in the glibc such as memcpy
> > > vector version and it is called very often by every process. So that
> > > we assume if the user program is running in a CPU with vector ISA
> > > would like to use vector by default. If we disable it by default and
> > > make it trigger the illegal instruction, that might be a burden since
> > > almost every process will use vector glibc memcpy or something like
> > > that.
> >
> > Do you have any evidence to support the assertion that almost every process
> > would use vector operations?  One could easily argue that the converse is
> > true: no existing software uses the vector extension now, so most likely a
> > process will not be using it.
> 
> Glibc ustreaming is just starting so you didn't see software using the
> vector extension now and this patchset is testing based on those
> optimized glibc too.
> Vincent Chen is working on the glibc vector support upstreaming and we
> will also upstream the vector version glibc memcpy, memcmp, memchr,
> memmove, memset, strcmp, strlen.
> Then we will see platform with vector support can use vector version
> mem* and str* functions automatically based on ifunc and platform
> without vector will use the original one automatically. These could be
> done to select the correct optimized glibc functions by ifunc
> mechanism.
> 
> >
> > >
> > > > Given the size of the vector state and potential power and performance
> > > > implications of enabling the vector engine, it seems like this should
> > > > treated similarly to Intel AMX on x86.  The full discussion of that is
> > > > here:
> > > >
> > > > https://lore.kernel.org/lkml/CALCETrW2QHa2TLvnUuVxAAheqcbSZ-5_WRXtDSAGcbG8N+gtdQ-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org/
> > > >
> > > > The cover letter for recent Intel AMX patches has a summary of the x86
> > > > implementation:
> > > >
> > > > https://lore.kernel.org/lkml/20210825155413.19673-1-chang.seok.bae@intel.com/
> > > >
> > > > If RISC-V were to adopt a similar approach, I think the significant
> > > > points are:
> > > >
> > > >   1. A process (or thread) must specifically request the desire to use
> > > > vector extensions (perhaps with some new arch_prctl() API),
> > > >
> > > >   2. The kernel is free to deny permission, perhaps based on
> > > > administrative rules or for other reasons, and
> > > >
> > > >   3. If a process attempts to use vector extensions before doing the
> > > > above, the process will die due to an illegal instruction.
> > >
> > > Thank you for sharing this, but I am not sure if we should treat
> > > vector like AMX on x86. IMHO, compiler might generate code with vector
> > > instructions automatically someday, maybe we should treat vector
> > > extensions like other extensions.
> > > If user knows the vector extension is supported in this CPU and he
> > > would like to use it, it seems we should let user use it directly just
> > > like other extensions.
> > > If user don't know it exists or not, user should use the library API
> > > transparently and let glibc or other library deal with it. The glibc
> > > ifunc feature or multi-lib should be able to choose the correct
> > > implementation.
> >
> > What makes me think that the vector extension should be treated like AMX is
> > that they both (1) have a significant amount of architectural state, and
> > (2) likely have a significant power and/or area impact on (non-emulated)
> > designs.
> >
> > For example, I think it is possible, maybe even likely, that vector
> > implementations will have one or more of the following behaviors:
> >
> >   1. A single vector unit shared among two or more harts,
> >
> >   2. Additional power consumption when the vector unit is enabled and idle
> > versus not being enabled at all,
> >
> >   3. For a system which supports variable operating frequency, a reduction
> > in the maximum frequency when the vector unit is enabled, and/or
> >
> >   4. The inability to enter low power states and/or delays to low power
> > states transitions when the vector unit is enabled.
> >
> > None of the above constraints apply to more ordinary extensions like
> > compressed or the various bit manipulation extensions.
> >
> > The discussion I linked to has some well reasoned arguments on why
> > substantial extensions should have a mechanism to request using them by
> > user space.  The discussion was in the context of Intel AMX, but applies to
> > further x86 extensions, and I think should also apply to similar extensions
> > on RISC-V, like vector here.
> 
> Have you ever checked the SVE/SVE2 of ARM64 implementation in Linux kernel too?
> IMHO, the vector of RISCV should be closer to the SVE2 of ARM64.

For SVE on arm64, memory is only allocated and the extension is only
enabled when a process is actively using it, which is not what this patch
set does.  If the memory allocation for state memory fails, it triggers a
BUG(); there is no graceful way to report this to the application.

To do something similar for RISC-V, you will need to write an illegal
instruction handler to retrieve the faulting instruction and partially
decode it enough to determine it is a vector instruction.  That seems
needlessly complicated, doesn't provide a way to gracefully report an
error if memory allocation fails, and doesn't provide any of the other
benefits that a defined API to request use of the vector extension would
provide.

Did you read the discussion on Intel AMX support that I previously linked
to?  There are well reasoned arguments why it is beneficial to require that
a process request access to substantial extensions, like RISC-V vector.

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