linux-kernel - Re: [PATCH v5 01/14] riscv: only use IPIs to handle cache-flushes on remote cpus

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Date:   Sat, 22 Jan 2022 09:40:23 +0530
From:   Anup Patel <anup@...infault.org>
To:     Heiko Stuebner <heiko@...ech.de>
Cc:     Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        DTML <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org List" <linux-kernel@...r.kernel.org>,
        Rob Herring <robh+dt@...nel.org>, Wei Fu <wefu@...hat.com>,
        liush <liush@...winnertech.com>, Guo Ren <guoren@...nel.org>,
        Atish Patra <atishp@...shpatra.org>,
        Drew Fustini <drew@...gleboard.org>,
        Christoph Hellwig <hch@....de>, Arnd Bergmann <arnd@...db.de>,
        Chen-Yu Tsai <wens@...e.org>,
        Maxime Ripard <maxime@...no.tech>,
        Daniel Lustig <dlustig@...dia.com>,
        Greg Favor <gfavor@...tanamicro.com>,
        Andrea Mondelli <andrea.mondelli@...wei.com>,
        Jonathan Behrens <behrensj@....edu>,
        Xinhaoqu <xinhaoqu@...wei.com>,
        Bill Huffman <huffman@...ence.com>,
        Nick Kossifidis <mick@....forth.gr>,
        Allen Baum <allen.baum@...erantotech.com>,
        Josh Scheid <jscheid@...tanamicro.com>,
        Richard Trauben <rtrauben@...il.com>,
        Samuel Holland <samuel@...lland.org>,
        Christoph Muellner <cmuellner@...ux.com>,
        Philipp Tomsich <philipp.tomsich@...ll.eu>
Subject: Re: [PATCH v5 01/14] riscv: only use IPIs to handle cache-flushes on
 remote cpus

On Fri, Jan 21, 2022 at 10:07 PM Heiko Stuebner <heiko@...ech.de> wrote:
>
> Right now, the flush_icache functions always use the SBI remote-fence
> when SBI is available, leaving using IPIs as a fallback mechanism.
>
> IPIs on the other hand are more flexible, as the ipi_ops are initially
> set to go through SBI but later will be overwritten to go through the
> ACLINT/CLINT.
>
> In a discussion we had, Nick was of the opinion that "In general we
> should prefer doing IPIs on S-mode through CLINT instead of going
> through SBI/M-mode, so IMHO we should only be using
> on_each_cpu_mask(ipi_remote_fence_i) on flush_icache_all()/
> flush_icache_mm() and remove any explicit calls to sbi_remote_fence_i(),
> because this way we continue using SBI for doing remote fences even after
> CLINT/ACLINT driver is registered, instead of using direct IPIs through
> CLINT/ACLINT."
>
> So follow this suggestion and just do ipi calls to have the proper kernel
> parts do them,
>
> This also fixes the null-ptr dereference happening when flush_icache_all()
> is called before sbi_init().
>
> Suggested-by: Nick Kossifidis <mick@....forth.gr>
> Signed-off-by: Heiko Stuebner <heiko@...ech.de>

For Guest/VM, only virtual IMSIC provides faster IPIs so in absence of
virtual IMSIC, the SBI IPI based IPIs are the best approach.

Like Atish mentioned, please base this work on the ACLINT series
because the ACLINT series adds required IPI infrastructure which helps
us select SBI IPI versus direct S-mode IPI based on hardware capability.

Regards,
Anup

> ---
>  arch/riscv/mm/cacheflush.c | 8 +-------
>  1 file changed, 1 insertion(+), 7 deletions(-)
>
> diff --git a/arch/riscv/mm/cacheflush.c b/arch/riscv/mm/cacheflush.c
> index 6cb7d96ad9c7..c35375cd52ec 100644
> --- a/arch/riscv/mm/cacheflush.c
> +++ b/arch/riscv/mm/cacheflush.c
> @@ -17,11 +17,7 @@ static void ipi_remote_fence_i(void *info)
>  void flush_icache_all(void)
>  {
>         local_flush_icache_all();
> -
> -       if (IS_ENABLED(CONFIG_RISCV_SBI))
> -               sbi_remote_fence_i(NULL);
> -       else
> -               on_each_cpu(ipi_remote_fence_i, NULL, 1);
> +       on_each_cpu(ipi_remote_fence_i, NULL, 1);
>  }
>  EXPORT_SYMBOL(flush_icache_all);
>
> @@ -66,8 +62,6 @@ void flush_icache_mm(struct mm_struct *mm, bool local)
>                  * with flush_icache_deferred().
>                  */
>                 smp_mb();
> -       } else if (IS_ENABLED(CONFIG_RISCV_SBI)) {
> -               sbi_remote_fence_i(&others);
>         } else {
>                 on_each_cpu_mask(&others, ipi_remote_fence_i, NULL, 1);
>         }
> --
> 2.30.2
>