[U-Boot] [PATCH 5/7] riscv: add support for multi-hart systems

Auer, Lukas lukas.auer at aisec.fraunhofer.de
Sun Mar 10 13:58:24 UTC 2019


Hi Rick,

On Thu, 2019-03-07 at 17:30 +0800, Rick Chen wrote:
> Hi Lukas
> 
> > > From: Lukas Auer [mailto:lukas.auer at aisec.fraunhofer.de]
> > > Sent: Tuesday, February 12, 2019 6:14 AM
> > > To: u-boot at lists.denx.de
> > > Cc: Atish Patra; Anup Patel; Bin Meng; Andreas Schwab; Palmer
> > > Dabbelt;
> > > Alexander Graf; Lukas Auer; Anup Patel; Rick Jian-Zhi Chen(陳建志);
> > > Baruch Siach;
> > > Stefan Roese
> > > Subject: [PATCH 5/7] riscv: add support for multi-hart systems
> > > 
> > > On RISC-V, all harts boot independently. To be able to run on a
> > > multi-hart system,
> > > U-Boot must be extended with the functionality to manage all
> > > harts in the
> > > system. A new config option, CONFIG_MAIN_HART, is used to select
> > > the hart
> > > U-Boot runs on. All other harts are halted.
> > > U-Boot can delegate functions to them using smp_call_function().
> > > 
> > > Every hart has a valid pointer to the global data structure and a
> > > 8KiB stack by
> > > default. The stack size is set with CONFIG_STACK_SIZE_SHIFT.
> > > 
> > > Signed-off-by: Lukas Auer <lukas.auer at aisec.fraunhofer.de>
> > > ---
> > > 
> > >  arch/riscv/Kconfig           |  12 +++++
> > >  arch/riscv/cpu/start.S       | 102
> > > ++++++++++++++++++++++++++++++++++-
> > >  arch/riscv/include/asm/csr.h |   1 +
> > >  3 files changed, 114 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig index
> > > 3a51339c4d..af8d0f8d67
> > > 100644
> > > --- a/arch/riscv/Kconfig
> > > +++ b/arch/riscv/Kconfig
> > > @@ -140,4 +140,16 @@ config SBI_IPI
> > >       default y if RISCV_SMODE
> > >       depends on SMP
> > > 
> > > +config MAIN_HART
> > > +     int "Main hart in system"
> > > +     default 0
> > > +     help
> > > +       Some SoCs include harts of various sizes, some of which
> > > might not
> > > +       be suitable for running U-Boot. CONFIG_MAIN_HART is used
> > > to select
> > > +       the hart U-Boot runs on.
> > > +
> > > +config STACK_SIZE_SHIFT
> > > +     int
> > > +     default 13
> > > +
> > >  endmenu
> > > diff --git a/arch/riscv/cpu/start.S b/arch/riscv/cpu/start.S
> > > index
> > > a30f6f7194..ce7230df37 100644
> > > --- a/arch/riscv/cpu/start.S
> > > +++ b/arch/riscv/cpu/start.S
> > > @@ -13,6 +13,7 @@
> > >  #include <config.h>
> > >  #include <common.h>
> > >  #include <elf.h>
> > > +#include <asm/csr.h>
> > >  #include <asm/encoding.h>
> > >  #include <generated/asm-offsets.h>
> > > 
> > > @@ -45,6 +46,23 @@ _start:
> > >       /* mask all interrupts */
> > >       csrw    MODE_PREFIX(ie), zero
> > > 
> > > +#ifdef CONFIG_SMP
> > > +     /* check if hart is within range */
> > > +     /* s0: hart id */
> > > +     li      t0, CONFIG_NR_CPUS
> > > +     bge     s0, t0, hart_out_of_bounds_loop
> > > +#endif
> > > +
> > > +#ifdef CONFIG_SMP
> > > +     /* set xSIE bit to receive IPIs */
> > > +#ifdef CONFIG_RISCV_MMODE
> > > +     li      t0, MIE_MSIE
> > > +#else
> > > +     li      t0, SIE_SSIE
> > > +#endif
> > > +     csrs    MODE_PREFIX(ie), t0
> > > +#endif
> > > +
> > >  /*
> > >   * Set stackpointer in internal/ex RAM to call board_init_f
> > >   */
> > > @@ -56,7 +74,25 @@ call_board_init_f:
> > >  call_board_init_f_0:
> > >       mv      a0, sp
> > >       jal     board_init_f_alloc_reserve
> > > +
> > > +     /*
> > > +      * Set global data pointer here for all harts,
> > > uninitialized at this
> > > +      * point.
> > > +      */
> > > +     mv      gp, a0
> > > +
> > > +     /* setup stack */
> > > +#ifdef CONFIG_SMP
> > > +     /* s0: hart id */
> > > +     slli    t0, s0, CONFIG_STACK_SIZE_SHIFT
> > > +     sub     sp, a0, t0
> > > +#else
> > >       mv      sp, a0
> > > +#endif
> > > +
> > > +     /* Continue on main hart, others branch to
> > > secondary_hart_loop */
> > > +     li      t0, CONFIG_MAIN_HART
> > > +     bne     s0, t0, secondary_hart_loop
> > > 
> > >       la      t0, prior_stage_fdt_address
> > >       SREG    s1, 0(t0)
> > > @@ -95,7 +131,14 @@ relocate_code:
> > >   *Set up the stack
> > >   */
> > >  stack_setup:
> > > +#ifdef CONFIG_SMP
> > > +     /* s0: hart id */
> > > +     slli    t0, s0, CONFIG_STACK_SIZE_SHIFT
> > > +     sub     sp, s2, t0
> > > +#else
> > >       mv      sp, s2
> > > +#endif
> > > +
> > >       la      t0, _start
> > >       sub     t6, s4, t0              /* t6 <- relocation offset
> > > */
> > >       beq     t0, s4, clear_bss       /* skip relocation */
> > > @@ -175,13 +218,30 @@ clear_bss:
> > >       add     t0, t0, t6              /* t0 <- rel __bss_start in
> > > RAM */
> > >       la      t1, __bss_end           /* t1 <- rel __bss_end in
> > > FLASH */
> > >       add     t1, t1, t6              /* t1 <- rel __bss_end in
> > > RAM */
> > > -     beq     t0, t1, call_board_init_r
> > > +     beq     t0, t1, relocate_secondary_harts
> > > 
> > >  clbss_l:
> > >       SREG    zero, 0(t0)             /* clear loop... */
> > >       addi    t0, t0, REGBYTES
> > >       bne     t0, t1, clbss_l
> > > 
> > > +relocate_secondary_harts:
> > > +#ifdef CONFIG_SMP
> > > +     /* send relocation IPI */
> > > +     la      t0, secondary_hart_relocate
> > > +     add     a0, t0, t6
> > > +
> > > +     /* store relocation offset */
> > > +     mv      s5, t6
> > > +
> > > +     mv      a1, s2
> > > +     mv      a2, s3
> > > +     jal     smp_call_function
> > > +
> > > +     /* restore relocation offset */
> > > +     mv      t6, s5
> > > +#endif
> > > +
> > >  /*
> > >   * We are done. Do not return, instead branch to second part of
> > > board
> > >   * initialization, now running from RAM.
> > > @@ -202,3 +262,43 @@ call_board_init_r:
> > >   * jump to it ...
> > >   */
> > >       jr      t4                      /* jump to board_init_r()
> > > */
> > > +
> > > +#ifdef CONFIG_SMP
> > > +hart_out_of_bounds_loop:
> > > +     /* Harts in this loop are out of bounds, increase
> > > CONFIG_NR_CPUS. */
> > > +     wfi
> > > +     j       hart_out_of_bounds_loop
> > > +#endif
> > > +
> > > +#ifdef CONFIG_SMP
> > > +/* SMP relocation entry */
> > > +secondary_hart_relocate:
> > > +     /* a1: new sp */
> > > +     /* a2: new gd */
> > > +     /* s0: hart id */
> > > +
> > > +     /* setup stack */
> > > +     slli    t0, s0, CONFIG_STACK_SIZE_SHIFT
> > > +     sub     sp, a1, t0
> > > +
> > > +     /* update global data pointer */
> > > +     mv      gp, a2
> > > +#endif
> > > +
> > > +secondary_hart_loop:
> > > +     wfi
> > > +
> > > +#ifdef CONFIG_SMP
> > > +     csrr    t0, MODE_PREFIX(ip)
> > > +#ifdef CONFIG_RISCV_MMODE
> > > +     andi    t0, t0, MIE_MSIE
> > > +#else
> > > +     andi    t0, t0, SIE_SSIE
> > > +#endif
> > > +     beqz    t0, secondary_hart_loop
> > > +
> > > +     mv      a0, s0
> > > +     jal     handle_ipi
> 
> I found that s0 maybe corrupted after execute handle_ipi.
> Because smp_function will be treated as a return function by
> compiler,
> so compiler will generate codes to execute restore after
> smp_function().
> 
> But actually it is a no-return function. So there maybe no chance to
> execute
> restore. And s0 will be corrupted somehow.
> 
> The usage of s0 in v2 flow seem the same as v1.
> So I reply mail in v1 patch.
> 

Thanks for reporting this issue!
What compiler version are you using? I never saw this issue with GCC
8.2.0, because optimization removes the ret following the call to
smp_function(), it is called using jr. The registers are therefore
restored before jumping to smp_function().

This system is probably too fragile. A simple solution would be to
store the hart ID somewhere else, for example register tp. What do you
think?

Thanks,
Lukas


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