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[v2,1/1] RISC-V: clarify the QEMU workaround in ISA parser

Message ID 8a127608cf6194a6d288289f2520bd1744b81437.1690350252.git.research_trasio@irq.a4lg.com (mailing list archive)
State Accepted
Commit 58473e8abbc68dad4a3c5f4fce6c6b32c85cbe28
Headers show
Series RISC-V: clarification of the QEMU workaround in the ISA parser | expand

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conchuod/verify_fixes success No Fixes tag
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Commit Message

Tsukasa OI July 26, 2023, 5:44 a.m. UTC 
From: Tsukasa OI <research_trasio@irq.a4lg.com>

Extensions prefixed with "Su" won't corrupt the workaround in many
cases.  The only exception is when the first multi-letter extension in the
ISA string begins with "Su" and is not prefixed with an underscore.

For instance, following ISA string can confuse this QEMU workaround.

*   "rv64imacsuclic" (RV64I + M + A + C + "Suclic")

However, this case is very unlikely because extensions prefixed by either
"Z", "Sm" or "Ss" will most likely precede first.

For instance, the "Suclic" extension (draft as of now) will be placed after
related "Smclic" and "Ssclic" extensions.  It's also highly likely that
other unprivileged extensions like "Zba" will precede.

It's also possible to suppress the issue in the QEMU workaround with an
underscore.  Following ISA string won't confuse the QEMU workaround.

*   "rv64imac_suclic" (RV64I + M + A + C + delimited "Suclic")

This fix is to tell kernel developers the nature of this workaround
precisely.  There are some "Su*" extensions to be ratified but don't worry
about this workaround too much.

This commit comes with other minor editorial fixes (for minor wording and
spacing issues, without changing the meaning).

Signed-off-by: Tsukasa OI <research_trasio@irq.a4lg.com>
---
 arch/riscv/kernel/cpufeature.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

Comments

Conor Dooley July 26, 2023, 6:56 a.m. UTC | #1 
On Wed, Jul 26, 2023 at 05:44:16AM +0000, Tsukasa OI wrote:
> From: Tsukasa OI <research_trasio@irq.a4lg.com>
> 
> Extensions prefixed with "Su" won't corrupt the workaround in many
> cases.  The only exception is when the first multi-letter extension in the
> ISA string begins with "Su" and is not prefixed with an underscore.
> 
> For instance, following ISA string can confuse this QEMU workaround.
> 
> *   "rv64imacsuclic" (RV64I + M + A + C + "Suclic")
> 
> However, this case is very unlikely because extensions prefixed by either
> "Z", "Sm" or "Ss" will most likely precede first.
> 
> For instance, the "Suclic" extension (draft as of now) will be placed after
> related "Smclic" and "Ssclic" extensions.  It's also highly likely that
> other unprivileged extensions like "Zba" will precede.
> 
> It's also possible to suppress the issue in the QEMU workaround with an
> underscore.  Following ISA string won't confuse the QEMU workaround.
> 
> *   "rv64imac_suclic" (RV64I + M + A + C + delimited "Suclic")
> 
> This fix is to tell kernel developers the nature of this workaround
> precisely.  There are some "Su*" extensions to be ratified but don't worry
> about this workaround too much.
> 
> This commit comes with other minor editorial fixes (for minor wording and
> spacing issues, without changing the meaning).
> 
> Signed-off-by: Tsukasa OI <research_trasio@irq.a4lg.com>

Pretty sure I gave you a Reviewed-by last time? Nevertheless,
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>

Thanks,
Conor.
diff mbox series

Patch

diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
index a8f66c015229..5b2ac109980c 100644
--- a/arch/riscv/kernel/cpufeature.c
+++ b/arch/riscv/kernel/cpufeature.c
@@ -169,10 +169,11 @@  void __init riscv_fill_hwcap(void)
 			switch (*ext) {
 			case 's':
 				/*
-				 * Workaround for invalid single-letter 's' & 'u'(QEMU).
-				 * No need to set the bit in riscv_isa as 's' & 'u' are
-				 * not valid ISA extensions. It works until multi-letter
-				 * extension starting with "Su" appears.
+				 * Workaround for invalid single-letters 's' & 'u' (QEMU).
+				 * No need to set the bits in riscv_isa as 's' and 'u' are
+				 * not valid ISA extensions. It works unless the first multi-letter
+				 * extension in the ISA string begins with "Su" and is not prefixed
+				 * with an underscore.
 				 */
 				if (ext[-1] != '_' && ext[1] == 'u') {
 					++isa;