linux-kernel - [PATCH v7 07/13] riscv: prevent compressed instructions in alternatives

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Message-Id: <20220307205310.1905628-8-heiko@sntech.de>
Date:   Mon,  7 Mar 2022 21:53:04 +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, linux-kernel@...r.kernel.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,
        gfavor@...tanamicro.com, andrea.mondelli@...wei.com,
        behrensj@....edu, xinhaoqu@...wei.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 v7 07/13] riscv: prevent compressed instructions in alternatives

Instructions are opportunistically compressed by the RISC-V assembler
when possible, but in alternatives-blocks both the old and new content
need to be the same size, so having the toolchain do somewhat random
optimizations will cause strange side-effects like
"attempt to move .org backwards" compile-time errors.

Already a simple "and" used in alternatives assembly will cause these
mismatched code sizes.

So prevent compressed instructions to be generated in alternatives-
code and use option-push and -pop to only limit this to the relevant
code blocks

Signed-off-by: Heiko Stuebner <heiko@...ech.de>
---
 arch/riscv/include/asm/alternative-macros.h | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/arch/riscv/include/asm/alternative-macros.h b/arch/riscv/include/asm/alternative-macros.h
index c0fb11fad631..3a52884bf23d 100644
--- a/arch/riscv/include/asm/alternative-macros.h
+++ b/arch/riscv/include/asm/alternative-macros.h
@@ -19,7 +19,10 @@
 	.popsection
 	.subsection 1
 888 :
+	.option push
+	.option norvc
 	\new_c
+	.option pop
 889 :
 	.previous
 	.org    . - (889b - 888b) + (887b - 886b)
@@ -29,7 +32,10 @@
 
 .macro __ALTERNATIVE_CFG old_c, new_c, vendor_id, errata_id, enable
 886 :
+	.option push
+	.option norvc
 	\old_c
+	.option pop
 887 :
 	ALT_NEW_CONTENT \vendor_id, \errata_id, \enable, \new_c
 .endm
@@ -40,7 +46,10 @@
 .macro __ALTERNATIVE_CFG_2 old_c, new_c_1, vendor_id_1, errata_id_1, enable_1, \
 				  new_c_2, vendor_id_2, errata_id_2, enable_2
 886 :
+	.option push
+	.option norvc
 	\old_c
+	.option pop
 887 :
 	ALT_NEW_CONTENT \vendor_id_1, \errata_id_1, \enable_1, \new_c_1
 	ALT_NEW_CONTENT \vendor_id_2, \errata_id_2, \enable_2, \new_c_2
@@ -70,7 +79,10 @@
 	".popsection\n"							\
 	".subsection 1\n"						\
 	"888 :\n"							\
+	".option push\n"						\
+	".option norvc\n"						\
 	new_c "\n"							\
+	".option pop\n"							\
 	"889 :\n"							\
 	".previous\n"							\
 	".org	. - (887b - 886b) + (889b - 888b)\n"			\
@@ -79,7 +91,10 @@
 
 #define __ALTERNATIVE_CFG(old_c, new_c, vendor_id, errata_id, enable)	\
 	"886 :\n"							\
+	".option push\n"						\
+	".option norvc\n"						\
 	old_c "\n"							\
+	".option pop\n"							\
 	"887 :\n"							\
 	ALT_NEW_CONTENT(vendor_id, errata_id, enable, new_c)
 
@@ -89,7 +104,10 @@
 #define __ALTERNATIVE_CFG_2(old_c, new_c_1, vendor_id_1, errata_id_1, enable_1, \
 				  new_c_2, vendor_id_2, errata_id_2, enable_2) \
 	"886 :\n"							\
+	".option push\n"						\
+	".option norvc\n"						\
 	old_c "\n"							\
+	".option pop\n"							\
 	"887 :\n"							\
 	ALT_NEW_CONTENT(vendor_id_1, errata_id_1, enable_1, new_c_1)	\
 	ALT_NEW_CONTENT(vendor_id_2, errata_id_2, enable_2, new_c_2)
-- 
2.30.2