Re: [PATCH 1/3] riscv: Enable cbo.zero only when all harts support Zicboz

[Deepak Gupta <debug@rivosinc.com>]

On Wed, Jun 05, 2024 at 01:56:45PM -0700, Samuel Holland wrote:
>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]
>Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
>---
>
> 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 5ef48cb20ee1..2879e26dbcd8 100644
>--- a/arch/riscv/kernel/cpufeature.c
>+++ b/arch/riscv/kernel/cpufeature.c
>@@ -27,6 +27,8 @@
>
> #define NUM_ALPHA_EXTS ('z' - 'a' + 1)
>
>+static bool any_cpu_has_zicboz;
>+
> unsigned long elf_hwcap __read_mostly;
>
> /* Host ISA bitmap */
>@@ -92,6 +94,7 @@ static bool riscv_isa_extension_check(int id)
> 			pr_err("Zicboz disabled as cboz-block-size present, but is not a power-of-2\n");
> 			return false;
> 		}
>+		any_cpu_has_zicboz = true;
> 		return true;
> 	case RISCV_ISA_EXT_INVALID:
> 		return false;
>@@ -724,8 +727,10 @@ 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");

`riscv_has_extension_unlikely` will check bitmap `riscv_isa[0]` which I think gets populated
by boot cpu (correct me if I am wrong here). So as long boot processor has the extension, it'll
try to set it on CPU which doesn't have it.

How about doing this

`riscv_fill_hwcap_from_isa_string` checks and enables bitmap for all CPUs.
So make a check there and if any of the CPU dont have `Zicboz`, then set a global bool
`zicboz_cpu_not_homogenous`.

Now in `riscv_user_isa_enable`, check following

If `zicboz_cpu_not_homogenous` is set, then you already detected that some of the CPUs don't
have support for `Zicboz` and thus you wouldn't set for CPU which even has the support and
print a warning message.

If `zicboz_cpu_not_homogenous` is clear, then that means all CPUs support the feature.
You simply enable it on hart.

> }
>
> #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);
>-- 
>2.44.1
>