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Message-ID: <ZuNyztqpH09ns0ws@ghost>
Date: Thu, 12 Sep 2024 16:01:34 -0700
From: Charlie Jenkins <charlie@...osinc.com>
To: Samuel Holland <samuel.holland@...ive.com>
Cc: linux-riscv@...ts.infradead.org, Palmer Dabbelt <palmer@...belt.com>,
Andrew Jones <ajones@...tanamicro.com>,
Conor Dooley <conor@...nel.org>, linux-kernel@...r.kernel.org,
Deepak Gupta <debug@...osinc.com>,
Conor Dooley <conor.dooley@...rochip.com>,
Albert Ou <aou@...s.berkeley.edu>, Andy Chiu <andy.chiu@...ive.com>,
Clément Léger <cleger@...osinc.com>,
Evan Green <evan@...osinc.com>,
Paul Walmsley <paul.walmsley@...ive.com>,
Sunil V L <sunilvl@...tanamicro.com>
Subject: Re: [PATCH v4 1/3] riscv: Enable cbo.zero only when all harts
support Zicboz
On Wed, Aug 14, 2024 at 01:10:54AM -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]
> Reviewed-by: Andrew Jones <ajones@...tanamicro.com>
> Reviewed-by: Conor Dooley <conor.dooley@...rochip.com>
> Reviewed-by: Deepak Gupta <debug@...osinc.com>
> Signed-off-by: Samuel Holland <samuel.holland@...ive.com>
> ---
I realized I was looking at v4 but responding to v3, whoops. I will put
my tags here as well.
Reviewed-by: Charlie Jenkins <charlie@...osinc.com>
Tested-by: Charlie Jenkins <charlie@...osinc.com>
>
> (no changes since v3)
>
> Changes in v3:
> - Rebase on riscv/for-next
>
> 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 b427188b28fc..0139d4ea8426 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;
> }
>
> @@ -918,8 +921,10 @@ 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);
> --
> 2.45.1
>
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