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

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Date:   Fri, 21 Jan 2022 17:36:05 +0100
From:   Heiko Stuebner <heiko@...ech.de>
To:     palmer@...belt.com, paul.walmsley@...ive.com, aou@...s.berkeley.edu
Cc:     linux-riscv@...ts.infradead.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, robh+dt@...nel.org, wefu@...hat.com,
        liush@...winnertech.com, guoren@...nel.org, atishp@...shpatra.org,
        anup@...infault.org, drew@...gleboard.org, hch@....de,
        arnd@...db.de, wens@...e.org, maxime@...no.tech,
        dlustig@...dia.com, gfavor@...tanamicro.com,
        andrea.mondelli@...wei.com, behrensj@....edu, xinhaoqu@...wei.com,
        huffman@...ence.com, mick@....forth.gr,
        allen.baum@...erantotech.com, jscheid@...tanamicro.com,
        rtrauben@...il.com, samuel@...lland.org, cmuellner@...ux.com,
        philipp.tomsich@...ll.eu, Heiko Stuebner <heiko@...ech.de>
Subject: [PATCH v5 01/14] riscv: only use IPIs to handle cache-flushes on remote cpus

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>
---
 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