linux-kernel - Re: [PATCH 0/8] riscv: improve boot time isa extensions handling

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Message-ID: <20221013162038.ehseju2neic2xu5z@kamzik>
Date:   Thu, 13 Oct 2022 18:20:38 +0200
From:   Andrew Jones <ajones@...tanamicro.com>
To:     Jisheng Zhang <jszhang@...nel.org>
Cc:     Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Anup Patel <apatel@...tanamicro.com>
Subject: Re: [PATCH 0/8] riscv: improve boot time isa extensions handling

On Thu, Oct 06, 2022 at 03:08:10PM +0800, Jisheng Zhang wrote:
> Generally, riscv ISA extensions are fixed for any specific hardware
> platform, that's to say, the hart features won't change any more
> after booting, this chacteristic make it straightforward to use
> static branch to check one specific ISA extension is supported or not
> to optimize performance.
> 
> However, some ISA extensions such as SVPBMT and ZICBOM are handled
> via. the alternative sequences.
> 
> Basically, for ease of maintenance, we prefer to use static branches
> in C code, but recently, Samuel found that the static branch usage in
> cpu_relax() breaks building with CONFIG_CC_OPTIMIZE_FOR_SIZE[1]. As
> Samuel pointed out, "Having a static branch in cpu_relax() is
> problematic because that function is widely inlined, including in some
> quite complex functions like in the VDSO. A quick measurement shows
> this static branch is responsible by itself for around 40% of the jump
> table."
> 
> Samuel's findings pointed out one of a few downsides of static branches
> usage in C code to handle ISA extensions detected at boot time:
> static branch's metadata in the __jump_table section, which is not
> discarded after ISA extensions are finalized, wastes some space.
> 
> I want to try to solve the issue for all possible dynamic handling of
> ISA extensions at boot time. Inspired by Mark[2], this patch introduces
> riscv_has_extension_*() helpers, which work like static branches but
> are patched using alternatives, thus the metadata can be freed after
> patching.
> 
> [1]https://lore.kernel.org/linux-riscv/20220922060958.44203-1-samuel@sholland.org/
> [2]https://lore.kernel.org/linux-arm-kernel/20220912162210.3626215-8-mark.rutland@arm.com/
> 
> 
> Jisheng Zhang (8):
>   riscv: move riscv_noncoherent_supported() out of ZICBOM probe
>   riscv: cpufeature: detect RISCV_ALTERNATIVES_EARLY_BOOT earlier
>   riscv: hwcap: make ISA extension ids can be used in asm
>   riscv: cpufeature: extend riscv_cpufeature_patch_func to all ISA
>     extensions
>   riscv: introduce riscv_has_extension_[un]likely()
>   riscv: fpu: switch has_fpu() to riscv_has_extension_likely()
>   riscv: cpu_relax: switch to riscv_has_extension_likely()
>   riscv: remove riscv_isa_ext_keys[] array and related usage
> 
>  arch/riscv/include/asm/errata_list.h    |  9 +--
>  arch/riscv/include/asm/hwcap.h          | 94 ++++++++++++++-----------
>  arch/riscv/include/asm/switch_to.h      |  3 +-
>  arch/riscv/include/asm/vdso/processor.h |  2 +-
>  arch/riscv/kernel/cpufeature.c          | 78 +++-----------------
>  arch/riscv/kernel/setup.c               |  4 ++
>  6 files changed, 71 insertions(+), 119 deletions(-)
> 
> -- 
> 2.37.2
>

This series also needs a KVM patch like below.

Thanks,
drew

>From 7069a6fa488ec4efad190884fe5fcf4a1c37753a Mon Sep 17 00:00:00 2001
From: Andrew Jones <ajones@...tanamicro.com>
Date: Thu, 13 Oct 2022 18:16:10 +0200
Subject: [PATCH] riscv: KVM: Switch has_svinval() to
 riscv_has_extension_unlikely()
Content-type: text/plain

Switch has_svinval() from static branch to the new helper
riscv_has_extension_unlikely().

Signed-off-by: Andrew Jones <ajones@...tanamicro.com>
---
 arch/riscv/kvm/tlb.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/arch/riscv/kvm/tlb.c b/arch/riscv/kvm/tlb.c
index 309d79b3e5cd..aa3da18ad873 100644
--- a/arch/riscv/kvm/tlb.c
+++ b/arch/riscv/kvm/tlb.c
@@ -15,8 +15,7 @@
 #include <asm/hwcap.h>
 #include <asm/insn-def.h>
 
-#define has_svinval()	\
-	static_branch_unlikely(&riscv_isa_ext_keys[RISCV_ISA_EXT_KEY_SVINVAL])
+#define has_svinval()	riscv_has_extension_unlikely(RISCV_ISA_EXT_SVINVAL)
 
 void kvm_riscv_local_hfence_gvma_vmid_gpa(unsigned long vmid,
 					  gpa_t gpa, gpa_t gpsz,
-- 
2.37.3