Merge patch series "RISC-V: Fixes for riscv_has_extension[un]likely()'s alternative dependency"

Conor Dooley <conor.dooley@microchip.com> says:

Here's my attempt at fixing both the use of an FPU on XIP kernels and
the issue that Jason ran into where CONFIG_FPU, which needs the
alternatives frame work for has_fpu() checks, could be enabled without
the alternatives actually being present.

For the former, a "slow" fallback that does not use alternatives is
added to riscv_has_extension_[un]likely() that can be used with XIP.
Obviously, we want to make use of Jisheng's alternatives based approach
where possible, so any users of riscv_has_extension_[un]likely() will
want to make sure that they select RISCV_ALTERNATIVE.
If they don't however, they'll hit the fallback path which (should,
sparing a silly mistake from me!) behave in the same way, thus
succeeding silently. Sounds like a

To prevent "depends on !XIP_KERNEL; select RISCV_ALTERNATIVE" spreading
like the plague through the various places that want to check for the
presence of extensions, and sidestep the potential silent "success"
mentioned above, all users RISCV_ALTERNATIVE are converted from selects
to dependencies, with the option being selected for all !XIP_KERNEL
builds.

I know that the VDSO was a key place that Jisheng wanted to use the new
helper rather than static branches, and I think the fallback path
should not cause issues there.

See the thread at [1] for the prior discussion.

1 - https://lore.kernel.org/linux-riscv/20230128172856.3814-1-jszhang@kernel.org/T/#m21390d570997145d31dd8bb95002fd61f99c6573

[Palmer: merging in the fixes as a branch as there's some features that
depend on it.]

* b4-shazam-merge:
  RISC-V: always select RISCV_ALTERNATIVE for non-xip kernels
  RISC-V: add non-alternative fallback for riscv_has_extension_[un]likely()

Link: https://lore.kernel.org/r/20230324100538.3514663-1-conor.dooley@microchip.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

arch/riscv/Kconfig

@@ -126,6 +126,7 @@ config RISCV
 	select OF_IRQ
 	select PCI_DOMAINS_GENERIC if PCI
 	select PCI_MSI if PCI
+	select RISCV_ALTERNATIVE if !XIP_KERNEL
 	select RISCV_INTC
 	select RISCV_TIMER if RISCV_SBI
 	select SIFIVE_PLIC
@@ -401,9 +402,8 @@ config RISCV_ISA_C
 config RISCV_ISA_SVPBMT
 	bool "SVPBMT extension support"
 	depends on 64BIT && MMU
-	depends on !XIP_KERNEL
+	depends on RISCV_ALTERNATIVE
 	default y
-	select RISCV_ALTERNATIVE
 	help
 	   Adds support to dynamically detect the presence of the SVPBMT
 	   ISA-extension (Supervisor-mode: page-based memory types) and
@@ -428,8 +428,8 @@ config TOOLCHAIN_HAS_ZBB
 config RISCV_ISA_ZBB
 	bool "Zbb extension support for bit manipulation instructions"
 	depends on TOOLCHAIN_HAS_ZBB
-	depends on !XIP_KERNEL && MMU
-	select RISCV_ALTERNATIVE
+	depends on MMU
+	depends on RISCV_ALTERNATIVE
 	default y
 	help
 	   Adds support to dynamically detect the presence of the ZBB
@@ -443,9 +443,9 @@ config RISCV_ISA_ZBB
 
 config RISCV_ISA_ZICBOM
 	bool "Zicbom extension support for non-coherent DMA operation"
-	depends on !XIP_KERNEL && MMU
+	depends on MMU
+	depends on RISCV_ALTERNATIVE
 	default y
-	select RISCV_ALTERNATIVE
 	select RISCV_DMA_NONCOHERENT
 	help
 	   Adds support to dynamically detect the presence of the ZICBOM

arch/riscv/Kconfig.erratas

@@ -2,8 +2,7 @@ menu "CPU errata selection"
 
 config ERRATA_SIFIVE
 	bool "SiFive errata"
-	depends on !XIP_KERNEL
-	select RISCV_ALTERNATIVE
+	depends on RISCV_ALTERNATIVE
 	help
 	  All SiFive errata Kconfig depend on this Kconfig. Disabling
 	  this Kconfig will disable all SiFive errata. Please say "Y"
@@ -35,8 +34,7 @@ config ERRATA_SIFIVE_CIP_1200
 
 config ERRATA_THEAD
 	bool "T-HEAD errata"
-	depends on !XIP_KERNEL
-	select RISCV_ALTERNATIVE
+	depends on RISCV_ALTERNATIVE
 	help
 	  All T-HEAD errata Kconfig depend on this Kconfig. Disabling
 	  this Kconfig will disable all T-HEAD errata. Please say "Y"

arch/riscv/include/asm/hwcap.h

@@ -57,18 +57,31 @@ struct riscv_isa_ext_data {
 	unsigned int isa_ext_id;
 };
 
+unsigned long riscv_isa_extension_base(const unsigned long *isa_bitmap);
+
+#define riscv_isa_extension_mask(ext) BIT_MASK(RISCV_ISA_EXT_##ext)
+
+bool __riscv_isa_extension_available(const unsigned long *isa_bitmap, int bit);
+#define riscv_isa_extension_available(isa_bitmap, ext)	\
+	__riscv_isa_extension_available(isa_bitmap, RISCV_ISA_EXT_##ext)
+
 static __always_inline bool
 riscv_has_extension_likely(const unsigned long ext)
 {
 	compiletime_assert(ext < RISCV_ISA_EXT_MAX,
 			   "ext must be < RISCV_ISA_EXT_MAX");
 
-	asm_volatile_goto(
-	ALTERNATIVE("j	%l[l_no]", "nop", 0, %[ext], 1)
-	:
-	: [ext] "i" (ext)
-	:
-	: l_no);
+	if (IS_ENABLED(CONFIG_RISCV_ALTERNATIVE)) {
+		asm_volatile_goto(
+		ALTERNATIVE("j	%l[l_no]", "nop", 0, %[ext], 1)
+		:
+		: [ext] "i" (ext)
+		:
+		: l_no);
+	} else {
+		if (!__riscv_isa_extension_available(NULL, ext))
+			goto l_no;
+	}
 
 	return true;
 l_no:
@@ -81,26 +94,23 @@ riscv_has_extension_unlikely(const unsigned long ext)
 	compiletime_assert(ext < RISCV_ISA_EXT_MAX,
 			   "ext must be < RISCV_ISA_EXT_MAX");
 
-	asm_volatile_goto(
-	ALTERNATIVE("nop", "j	%l[l_yes]", 0, %[ext], 1)
-	:
-	: [ext] "i" (ext)
-	:
-	: l_yes);
+	if (IS_ENABLED(CONFIG_RISCV_ALTERNATIVE)) {
+		asm_volatile_goto(
+		ALTERNATIVE("nop", "j	%l[l_yes]", 0, %[ext], 1)
+		:
+		: [ext] "i" (ext)
+		:
+		: l_yes);
+	} else {
+		if (__riscv_isa_extension_available(NULL, ext))
+			goto l_yes;
+	}
 
 	return false;
 l_yes:
 	return true;
 }
 
-unsigned long riscv_isa_extension_base(const unsigned long *isa_bitmap);
-
-#define riscv_isa_extension_mask(ext) BIT_MASK(RISCV_ISA_EXT_##ext)
-
-bool __riscv_isa_extension_available(const unsigned long *isa_bitmap, int bit);
-#define riscv_isa_extension_available(isa_bitmap, ext)	\
-	__riscv_isa_extension_available(isa_bitmap, RISCV_ISA_EXT_##ext)
-
 #endif
 
 #endif /* _ASM_RISCV_HWCAP_H */