Re: [PATCH v3 2/2] RISC-V: Clean up the Zicbom block size probing

["Heiko Stübner" <heiko@sntech.de>]

Am Donnerstag, 8. September 2022, 09:11:57 CEST schrieb Andrew Jones:
> On Wed, Sep 07, 2022 at 03:47:09PM -0700, Atish Patra wrote:
> > On Tue, Sep 6, 2022 at 12:45 AM Andrew Jones <ajones@ventanamicro.com>
> > wrote:
> > 
> > > From: Palmer Dabbelt <palmer@rivosinc.com>
> > >
> > > This fixes two issues: I truncated the warning's hart ID when porting to
> > > the 64-bit hart ID code, and the original code's warning handling could
> > > fire on an uninitialized hart ID.
> > >
> > > The biggest change here is that riscv_cbom_block_size is no longer
> > > initialized, as IMO the default isn't sane: there's nothing in the ISA
> > > that mandates any specific cache block size, so falling back to one will
> > > just silently produce the wrong answer on some systems.  This also
> > > changes the probing order so the cache block size is known before
> > > enabling Zicbom support.
> > >
> > > Fixes: 3aefb2ee5bdd ("riscv: implement Zicbom-based CMO instructions + the
> > > t-head variant")
> > > Fixes: 1631ba1259d6 ("riscv: Add support for non-coherent devices using
> > > zicbom extension")
> > > Reported-by: kernel test robot <lkp@intel.com>
> > > Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>
> > > Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
> > > [Rebased on Anup's move patch and applied Conor Dooley's and Heiko
> > >  Stuebner's changes.]
> > > Signed-off-by: Andrew Jones <ajones@ventanamicro.com>
> > > ---
> > >  arch/riscv/errata/thead/errata.c |  1 +
> > >  arch/riscv/kernel/setup.c        |  2 +-
> > >  arch/riscv/mm/cacheflush.c       | 21 +++++++++++----------
> > >  arch/riscv/mm/dma-noncoherent.c  |  2 ++
> > >  4 files changed, 15 insertions(+), 11 deletions(-)
> > >
> > > diff --git a/arch/riscv/errata/thead/errata.c
> > > b/arch/riscv/errata/thead/errata.c
> > > index 202c83f677b2..96648c176f37 100644
> > > --- a/arch/riscv/errata/thead/errata.c
> > > +++ b/arch/riscv/errata/thead/errata.c
> > > @@ -37,6 +37,7 @@ static bool errata_probe_cmo(unsigned int stage,
> > >         if (stage == RISCV_ALTERNATIVES_EARLY_BOOT)
> > >                 return false;
> > >
> > > +       riscv_cbom_block_size = L1_CACHE_BYTES;
> > >         riscv_noncoherent_supported();
> > >         return true;
> > >  #else
> > > diff --git a/arch/riscv/kernel/setup.c b/arch/riscv/kernel/setup.c
> > > index 95ef6e2bf45c..2dfc463b86bb 100644
> > > --- a/arch/riscv/kernel/setup.c
> > > +++ b/arch/riscv/kernel/setup.c
> > > @@ -296,8 +296,8 @@ void __init setup_arch(char **cmdline_p)
> > >         setup_smp();
> > >  #endif
> > >
> > > -       riscv_fill_hwcap();
> > >         riscv_init_cbom_blocksize();
> > > +       riscv_fill_hwcap();
> > >         apply_boot_alternatives();
> > >  }
> > >
> > > diff --git a/arch/riscv/mm/cacheflush.c b/arch/riscv/mm/cacheflush.c
> > > index 336c5deea870..e5b087be1577 100644
> > > --- a/arch/riscv/mm/cacheflush.c
> > > +++ b/arch/riscv/mm/cacheflush.c
> > > @@ -89,39 +89,40 @@ void flush_icache_pte(pte_t pte)
> > >  }
> > >  #endif /* CONFIG_MMU */
> > >
> > > -unsigned int riscv_cbom_block_size = L1_CACHE_BYTES;
> > > +unsigned int riscv_cbom_block_size;
> > >
> > >  #ifdef CONFIG_RISCV_ISA_ZICBOM
> > >  void riscv_init_cbom_blocksize(void)
> > >  {
> > >         struct device_node *node;
> > > +       unsigned long cbom_hartid;
> > > +       u32 val, probed_block_size;
> > >         int ret;
> > > -       u32 val;
> > >
> > > +       probed_block_size = 0;
> > >         for_each_of_cpu_node(node) {
> > >                 unsigned long hartid;
> > > -               int cbom_hartid;
> > >
> > >                 ret = riscv_of_processor_hartid(node, &hartid);
> > >                 if (ret)
> > >                         continue;
> > >
> > > -               if (hartid < 0)
> > > -                       continue;
> > > -
> > >                 /* set block-size for cbom extension if available */
> > >                 ret = of_property_read_u32(node, "riscv,cbom-block-size",
> > > &val);
> > >                 if (ret)
> > >                         continue;
> > >
> > > -               if (!riscv_cbom_block_size) {
> > > -                       riscv_cbom_block_size = val;
> > > +               if (!probed_block_size) {
> > > +                       probed_block_size = val;
> > >                         cbom_hartid = hartid;
> > >                 } else {
> > > -                       if (riscv_cbom_block_size != val)
> > > -                               pr_warn("cbom-block-size mismatched
> > > between harts %d and %lu\n",
> > > +                       if (probed_block_size != val)
> > > +                               pr_warn("cbom-block-size mismatched
> > > between harts %lu and %lu\n",
> > >                                         cbom_hartid, hartid);
> > >
> > 
> > Maybe add more info saying the first one will be selected in that case as
> > it is just a warning.
> 
> If we detect a mismatch then should we disable the CMO extension?

From a user's pov I'd think their system might stop working with disabled
cmo - for things like networking / mass storage or so.

Also the amount of misbehaviour might depend on weather the value is
shrinking or expanding.

Going from block_size x -> x/2 will "just" result in some areas being
handled twice, where going from x -> 2x will leave out some areas,
when the cpu itself still just does "x" .


An experiment on the D1 supports that thought ;-)
with L1_CACHE_BYTES / 2, networking keeps working
with L1_CACHE_BYTES * 2 (plus adapting MINALIGN) breaks networking.


So I'd think, we should loudly warn about misconfiguration anyway,
but could just use the smallest value as block_size (in a future patch)
to keep the most amounts of systems running in such a case.


Heiko


> The
> current spec says "size of a cache block shall be uniform throughout the
> system", even though that may be relaxed in later extensions. I also now
> recall you suggested that when DT parsing ends up with a zero block size
> we should disable the CMO extension. riscv_init_cbom_blocksize() still
> doesn't handle zeros in the DT correctly, even with this patch, though.
> For example, if the first harts parsed are zero, but the latter harts
> all match, we won't even get the mismatch warning.
> 
> But, for now, my preference would be to merge this patch, as it's an
> improvement on its own. I can try to write another patch which handles
> zeros and mismatches by disabling the feature. What do you think?
> 
> Thanks,
> drew
> 
> > 
> > 
> > >                 }
> > >         }
> > > +
> > > +       if (probed_block_size)
> > > +               riscv_cbom_block_size = probed_block_size;
> > >  }
> > >  #endif
> > > diff --git a/arch/riscv/mm/dma-noncoherent.c
> > > b/arch/riscv/mm/dma-noncoherent.c
> > > index 3f502a1a68b1..d919efab6eba 100644
> > > --- a/arch/riscv/mm/dma-noncoherent.c
> > > +++ b/arch/riscv/mm/dma-noncoherent.c
> > > @@ -74,5 +74,7 @@ void arch_setup_dma_ops(struct device *dev, u64
> > > dma_base, u64 size,
> > >
> > >  void riscv_noncoherent_supported(void)
> > >  {
> > > +       WARN(!riscv_cbom_block_size,
> > > +            "Non-coherent DMA support enabled without a block size\n");
> > >         noncoherent_supported = true;
> > >  }
> > > --
> > > 2.37.2
> > >
> > >
> > Otherwise, LGTM.
> > 
> > Reviewed-by: Atish Patra <atishp@rivosinc.com>
> > 
> 





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