From 1b57747e978f920fb2affd1952ed913276019115 Mon Sep 17 00:00:00 2001 From: Samuel Holland Date: Wed, 14 Aug 2024 01:10:54 -0700 Subject: [PATCH 1/3] riscv: Enable cbo.zero only when all harts support Zicboz Currently, we enable cbo.zero for usermode on each hart that supports the Zicboz extension. This means that the [ms]envcfg CSR value may differ between harts. Other features, such as pointer masking and CFI, require setting [ms]envcfg bits on a per-thread basis. The combination of these two adds quite some complexity and overhead to context switching, as we would need to maintain two separate masks for the per-hart and per-thread bits. Andrew Jones, who originally added Zicboz support, writes[1][2]: I've approached Zicboz the same way I would approach all extensions, which is to be per-hart. I'm not currently aware of a platform that is / will be composed of harts where some have Zicboz and others don't, but there's nothing stopping a platform like that from being built. So, how about we add code that confirms Zicboz is on all harts. If any hart does not have it, then we complain loudly and disable it on all the other harts. If it was just a hardware description bug, then it'll get fixed. If there's actually a platform which doesn't have Zicboz on all harts, then, when the issue is reported, we can decide to not support it, support it with defconfig, or support it under a Kconfig guard which must be enabled by the user. Let's follow his suggested solution and require the extension to be available on all harts, so the envcfg CSR value does not need to change when a thread migrates between harts. Since we are doing this for all extensions with fields in envcfg, the CSR itself only needs to be saved/ restored when it is present on all harts. This should not be a regression as no known hardware has asymmetric Zicboz support, but if anyone reports seeing the warning, we will re-evaluate our solution. Link: https://lore.kernel.org/linux-riscv/20240322-168f191eeb8479b2ea169a5e@orel/ [1] Link: https://lore.kernel.org/linux-riscv/20240323-28943722feb57a41fb0ff488@orel/ [2] Reviewed-by: Andrew Jones Reviewed-by: Conor Dooley Reviewed-by: Deepak Gupta Signed-off-by: Samuel Holland Reviewed-by: Charlie Jenkins Tested-by: Charlie Jenkins Link: https://lore.kernel.org/r/20240814081126.956287-2-samuel.holland@sifive.com Signed-off-by: Palmer Dabbelt --- arch/riscv/kernel/cpufeature.c | 7 ++++++- arch/riscv/kernel/suspend.c | 4 ++-- 2 files changed, 8 insertions(+), 3 deletions(-) diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c index 3a8eeaa9310c..e560a253e99b 100644 --- a/arch/riscv/kernel/cpufeature.c +++ b/arch/riscv/kernel/cpufeature.c @@ -28,6 +28,8 @@ #define NUM_ALPHA_EXTS ('z' - 'a' + 1) +static bool any_cpu_has_zicboz; + unsigned long elf_hwcap __read_mostly; /* Host ISA bitmap */ @@ -98,6 +100,7 @@ static int riscv_ext_zicboz_validate(const struct riscv_isa_ext_data *data, pr_err("Zicboz disabled as cboz-block-size present, but is not a power-of-2\n"); return -EINVAL; } + any_cpu_has_zicboz = true; return 0; } @@ -919,8 +922,10 @@ unsigned long riscv_get_elf_hwcap(void) void riscv_user_isa_enable(void) { - if (riscv_cpu_has_extension_unlikely(smp_processor_id(), RISCV_ISA_EXT_ZICBOZ)) + if (riscv_has_extension_unlikely(RISCV_ISA_EXT_ZICBOZ)) csr_set(CSR_ENVCFG, ENVCFG_CBZE); + else if (any_cpu_has_zicboz) + pr_warn_once("Zicboz disabled as it is unavailable on some harts\n"); } #ifdef CONFIG_RISCV_ALTERNATIVE diff --git a/arch/riscv/kernel/suspend.c b/arch/riscv/kernel/suspend.c index c8cec0cc5833..9a8a0dc035b2 100644 --- a/arch/riscv/kernel/suspend.c +++ b/arch/riscv/kernel/suspend.c @@ -14,7 +14,7 @@ void suspend_save_csrs(struct suspend_context *context) { - if (riscv_cpu_has_extension_unlikely(smp_processor_id(), RISCV_ISA_EXT_XLINUXENVCFG)) + if (riscv_has_extension_unlikely(RISCV_ISA_EXT_XLINUXENVCFG)) context->envcfg = csr_read(CSR_ENVCFG); context->tvec = csr_read(CSR_TVEC); context->ie = csr_read(CSR_IE); @@ -37,7 +37,7 @@ void suspend_save_csrs(struct suspend_context *context) void suspend_restore_csrs(struct suspend_context *context) { csr_write(CSR_SCRATCH, 0); - if (riscv_cpu_has_extension_unlikely(smp_processor_id(), RISCV_ISA_EXT_XLINUXENVCFG)) + if (riscv_has_extension_unlikely(RISCV_ISA_EXT_XLINUXENVCFG)) csr_write(CSR_ENVCFG, context->envcfg); csr_write(CSR_TVEC, context->tvec); csr_write(CSR_IE, context->ie); From 5fc7355f01376e69964bb21b685025b042c37acc Mon Sep 17 00:00:00 2001 From: Samuel Holland Date: Wed, 14 Aug 2024 01:10:55 -0700 Subject: [PATCH 2/3] riscv: Add support for per-thread envcfg CSR values Some bits in the [ms]envcfg CSR, such as the CFI state and pointer masking mode, need to be controlled on a per-thread basis. Support this by keeping a copy of the CSR value in struct thread_struct and writing it during context switches. It is safe to discard the old CSR value during the context switch because the CSR is modified only by software, so the CSR will remain in sync with the copy in thread_struct. Use ALTERNATIVE directly instead of riscv_has_extension_unlikely() to minimize branchiness in the context switching code. Since thread_struct is copied during fork(), setting the value for the init task sets the default value for all other threads. Reviewed-by: Andrew Jones Reviewed-by: Deepak Gupta Signed-off-by: Samuel Holland Reviewed-by: Charlie Jenkins Link: https://lore.kernel.org/r/20240814081126.956287-3-samuel.holland@sifive.com Signed-off-by: Palmer Dabbelt --- arch/riscv/include/asm/processor.h | 1 + arch/riscv/include/asm/switch_to.h | 8 ++++++++ arch/riscv/kernel/cpufeature.c | 2 +- 3 files changed, 10 insertions(+), 1 deletion(-) diff --git a/arch/riscv/include/asm/processor.h b/arch/riscv/include/asm/processor.h index efa1b3519b23..c1a492508835 100644 --- a/arch/riscv/include/asm/processor.h +++ b/arch/riscv/include/asm/processor.h @@ -102,6 +102,7 @@ struct thread_struct { unsigned long s[12]; /* s[0]: frame pointer */ struct __riscv_d_ext_state fstate; unsigned long bad_cause; + unsigned long envcfg; u32 riscv_v_flags; u32 vstate_ctrl; struct __riscv_v_ext_state vstate; diff --git a/arch/riscv/include/asm/switch_to.h b/arch/riscv/include/asm/switch_to.h index 7594df37cc9f..9685cd85e57c 100644 --- a/arch/riscv/include/asm/switch_to.h +++ b/arch/riscv/include/asm/switch_to.h @@ -70,6 +70,13 @@ static __always_inline bool has_fpu(void) { return false; } #define __switch_to_fpu(__prev, __next) do { } while (0) #endif +static inline void __switch_to_envcfg(struct task_struct *next) +{ + asm volatile (ALTERNATIVE("nop", "csrw " __stringify(CSR_ENVCFG) ", %0", + 0, RISCV_ISA_EXT_XLINUXENVCFG, 1) + :: "r" (next->thread.envcfg) : "memory"); +} + extern struct task_struct *__switch_to(struct task_struct *, struct task_struct *); @@ -103,6 +110,7 @@ do { \ __switch_to_vector(__prev, __next); \ if (switch_to_should_flush_icache(__next)) \ local_flush_icache_all(); \ + __switch_to_envcfg(__next); \ ((last) = __switch_to(__prev, __next)); \ } while (0) diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c index e560a253e99b..27bafc5dd62d 100644 --- a/arch/riscv/kernel/cpufeature.c +++ b/arch/riscv/kernel/cpufeature.c @@ -923,7 +923,7 @@ unsigned long riscv_get_elf_hwcap(void) void riscv_user_isa_enable(void) { if (riscv_has_extension_unlikely(RISCV_ISA_EXT_ZICBOZ)) - csr_set(CSR_ENVCFG, ENVCFG_CBZE); + current->thread.envcfg |= ENVCFG_CBZE; else if (any_cpu_has_zicboz) pr_warn_once("Zicboz disabled as it is unavailable on some harts\n"); } From 368546ebe7e74cb6e18f17768533ab7077392a8c Mon Sep 17 00:00:00 2001 From: Samuel Holland Date: Wed, 14 Aug 2024 01:10:56 -0700 Subject: [PATCH 3/3] riscv: Call riscv_user_isa_enable() only on the boot hart Now that the [ms]envcfg CSR value is maintained per thread, not per hart, riscv_user_isa_enable() only needs to be called once during boot, to set the value for the init task. This also allows it to be marked as __init. Reviewed-by: Andrew Jones Reviewed-by: Conor Dooley Reviewed-by: Deepak Gupta Signed-off-by: Samuel Holland Reviewed-by: Charlie Jenkins Link: https://lore.kernel.org/r/20240814081126.956287-4-samuel.holland@sifive.com Signed-off-by: Palmer Dabbelt --- arch/riscv/include/asm/cpufeature.h | 2 +- arch/riscv/kernel/cpufeature.c | 4 ++-- arch/riscv/kernel/smpboot.c | 2 -- 3 files changed, 3 insertions(+), 5 deletions(-) diff --git a/arch/riscv/include/asm/cpufeature.h b/arch/riscv/include/asm/cpufeature.h index 45f9c1171a48..ce9a995730c1 100644 --- a/arch/riscv/include/asm/cpufeature.h +++ b/arch/riscv/include/asm/cpufeature.h @@ -31,7 +31,7 @@ DECLARE_PER_CPU(struct riscv_cpuinfo, riscv_cpuinfo); /* Per-cpu ISA extensions. */ extern struct riscv_isainfo hart_isa[NR_CPUS]; -void riscv_user_isa_enable(void); +void __init riscv_user_isa_enable(void); #define _RISCV_ISA_EXT_DATA(_name, _id, _subset_exts, _subset_exts_size, _validate) { \ .name = #_name, \ diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c index 27bafc5dd62d..b3a057c36996 100644 --- a/arch/riscv/kernel/cpufeature.c +++ b/arch/riscv/kernel/cpufeature.c @@ -920,12 +920,12 @@ unsigned long riscv_get_elf_hwcap(void) return hwcap; } -void riscv_user_isa_enable(void) +void __init riscv_user_isa_enable(void) { if (riscv_has_extension_unlikely(RISCV_ISA_EXT_ZICBOZ)) current->thread.envcfg |= ENVCFG_CBZE; else if (any_cpu_has_zicboz) - pr_warn_once("Zicboz disabled as it is unavailable on some harts\n"); + pr_warn("Zicboz disabled as it is unavailable on some harts\n"); } #ifdef CONFIG_RISCV_ALTERNATIVE diff --git a/arch/riscv/kernel/smpboot.c b/arch/riscv/kernel/smpboot.c index 0f8f1c95ac38..e36d20205bd7 100644 --- a/arch/riscv/kernel/smpboot.c +++ b/arch/riscv/kernel/smpboot.c @@ -233,8 +233,6 @@ asmlinkage __visible void smp_callin(void) numa_add_cpu(curr_cpuid); set_cpu_online(curr_cpuid, true); - riscv_user_isa_enable(); - /* * Remote cache and TLB flushes are ignored while the CPU is offline, * so flush them both right now just in case.