linux-kernel - Re: [PATCH v6 1/1] riscv: mm: Add support for Svinval extension

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Date: Fri, 21 Jun 2024 11:15:52 +0200
From: Andrew Jones <ajones@...tanamicro.com>
To: Mayuresh Chitale <mchitale@...tanamicro.com>
Cc: linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org, 
	Paul Walmsley <paul.walmsley@...ive.com>, Palmer Dabbelt <palmer@...belt.com>, 
	Albert Ou <aou@...s.berkeley.edu>, Alexandre Ghiti <alexghiti@...osinc.com>, 
	Samuel Holland <samuel.holland@...ive.com>
Subject: Re: [PATCH v6 1/1] riscv: mm: Add support for Svinval extension

On Sun, Jun 09, 2024 at 04:51:03PM GMT, Mayuresh Chitale wrote:
> The Svinval extension splits SFENCE.VMA instruction into finer-grained
> invalidation and ordering operations and is mandatory for RVA23S64 profile.
> When Svinval is enabled the local_flush_tlb_range_threshold_asid function
> should use the following sequence to optimize the tlb flushes instead of
> a simple sfence.vma:
> 
> sfence.w.inval
> svinval.vma
>   .
>   .
> svinval.vma
> sfence.inval.ir
> 
> The maximum number of consecutive svinval.vma instructions that
> can be executed in local_flush_tlb_range_threshold_asid function
> is limited to 64. This is required to avoid soft lockups and the
> approach is similar to that used in arm64.
> 
> Signed-off-by: Mayuresh Chitale <mchitale@...tanamicro.com>
> ---
>  arch/riscv/mm/tlbflush.c | 58 ++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 58 insertions(+)
> 
> diff --git a/arch/riscv/mm/tlbflush.c b/arch/riscv/mm/tlbflush.c
> index 9b6e86ce3867..49d7978ac8d3 100644
> --- a/arch/riscv/mm/tlbflush.c
> +++ b/arch/riscv/mm/tlbflush.c
> @@ -6,6 +6,54 @@
>  #include <linux/hugetlb.h>
>  #include <asm/sbi.h>
>  #include <asm/mmu_context.h>
> +#include <asm/cpufeature.h>
> +
> +#define has_svinval()	riscv_has_extension_unlikely(RISCV_ISA_EXT_SVINVAL)
> +
> +static inline void local_sfence_inval_ir(void)
> +{
> +	/*
> +	 * SFENCE.INVAL.IR
> +	 * 0001100 00001 00000 000 00000 1110011
> +	 */
> +	__asm__ __volatile__ (".word 0x18100073" ::: "memory");
> +}
> +
> +static inline void local_sfence_w_inval(void)
> +{
> +	/*
> +	 * SFENCE.W.INVAL
> +	 * 0001100 00000 00000 000 00000 1110011
> +	 */
> +	__asm__ __volatile__ (".word 0x18000073" ::: "memory");
> +}
> +
> +static inline void local_sinval_vma_asid(unsigned long vma, unsigned long asid)

Please name this to

  void local_sinval_vma(unsigned long vaddr, unsigned long asid)

to match the spec's naming. But, if we want the arguments in the name,
then it should be something like

  void local_sinval_vma_addr_asid(unsigned long vaddr, unsigned long asid)

but I think that's unnecessary.

> +{
> +	if (asid != FLUSH_TLB_NO_ASID) {
> +		/*
> +		 * rs1 = a0 (VMA)
> +		 * rs2 = a1 (asid)
> +		 * SINVAL.VMA a0, a1
> +		 * 0001011 01011 01010 000 00000 1110011
> +		 */
> +		__asm__ __volatile__ ("add a0, %0, zero\n"
> +					"add a1, %1, zero\n"
> +					".word 0x16B50073\n"
> +					:: "r" (vma), "r" (asid)
> +					: "a0", "a1", "memory");
> +	} else {
> +		/*
> +		 * rs1 = a0 (VMA)
> +		 * rs2 = 0
> +		 * SINVAL.VMA a0
> +		 * 0001011 00000 01010 000 00000 1110011
> +		 */
> +		__asm__ __volatile__ ("add a0, %0, zero\n"
> +					".word 0x16050073\n"
> +					:: "r" (vma) : "a0", "memory");
> +	}

Please create an SINVAL_VMA instruction in insn-def.h to allow the
compiler to choose which registers will be used for asid and vaddr. And,
since SINVAL_VMA will be in insn-def.h, then we might as well add
SFENCE_INVAL_IR and SFENCE_W_INVAL there too for consistency, even though
they don't have operands. We should still create the three static inline
wrapper functions here though.

> +}
>  
>  /*
>   * Flush entire TLB if number of entries to be flushed is greater
> @@ -26,6 +74,16 @@ static void local_flush_tlb_range_threshold_asid(unsigned long start,
>  		return;
>  	}
>  
> +	if (has_svinval()) {
> +		local_sfence_w_inval();
> +		for (i = 0; i < nr_ptes_in_range; ++i) {

Where do we limit this to 64 as the commit message states it does?

> +			local_sinval_vma_asid(start, asid);
> +			start += stride;
> +		}
> +		local_sfence_inval_ir();
> +		return;
> +	}
> +
>  	for (i = 0; i < nr_ptes_in_range; ++i) {
>  		local_flush_tlb_page_asid(start, asid);
>  		start += stride;
> -- 
> 2.34.1
>

Thanks,
drew