linux-kernel - Re: [PATCH v2 05/12] RISC-V: KVM: Don't flush TLB when PTE is unchanged

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Message-ID: <ffd04c9b-030d-4eaa-bbf6-d1f416565c35@linux.dev>
Date: Fri, 13 Jun 2025 17:09:41 -0700
From: Atish Patra <atish.patra@...ux.dev>
To: Anup Patel <apatel@...tanamicro.com>
Cc: Palmer Dabbelt <palmer@...belt.com>,
 Paul Walmsley <paul.walmsley@...ive.com>, Alexandre Ghiti <alex@...ti.fr>,
 Andrew Jones <ajones@...tanamicro.com>, Anup Patel <anup@...infault.org>,
 kvm@...r.kernel.org, kvm-riscv@...ts.infradead.org,
 linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 05/12] RISC-V: KVM: Don't flush TLB when PTE is
 unchanged


On 6/12/25 11:57 PM, Anup Patel wrote:
> The gstage_set_pte() and gstage_op_pte() should flush TLB only when
> a leaf PTE changes so that unnecessary TLB flushes can be avoided.
>
> Signed-off-by: Anup Patel <apatel@...tanamicro.com>
> ---
>   arch/riscv/kvm/mmu.c | 14 +++++++++-----
>   1 file changed, 9 insertions(+), 5 deletions(-)
>
> diff --git a/arch/riscv/kvm/mmu.c b/arch/riscv/kvm/mmu.c
> index 1087ea74567b..29f1bd853a66 100644
> --- a/arch/riscv/kvm/mmu.c
> +++ b/arch/riscv/kvm/mmu.c
> @@ -167,9 +167,11 @@ static int gstage_set_pte(struct kvm *kvm, u32 level,
>   		ptep = &next_ptep[gstage_pte_index(addr, current_level)];
>   	}
>   
> -	set_pte(ptep, *new_pte);
> -	if (gstage_pte_leaf(ptep))
> -		gstage_remote_tlb_flush(kvm, current_level, addr);
> +	if (pte_val(*ptep) != pte_val(*new_pte)) {
> +		set_pte(ptep, *new_pte);
> +		if (gstage_pte_leaf(ptep))
> +			gstage_remote_tlb_flush(kvm, current_level, addr);
> +	}
>   
>   	return 0;
>   }
> @@ -229,7 +231,7 @@ static void gstage_op_pte(struct kvm *kvm, gpa_t addr,
>   			  pte_t *ptep, u32 ptep_level, enum gstage_op op)
>   {
>   	int i, ret;
> -	pte_t *next_ptep;
> +	pte_t old_pte, *next_ptep;
>   	u32 next_ptep_level;
>   	unsigned long next_page_size, page_size;
>   
> @@ -258,11 +260,13 @@ static void gstage_op_pte(struct kvm *kvm, gpa_t addr,
>   		if (op == GSTAGE_OP_CLEAR)
>   			put_page(virt_to_page(next_ptep));
>   	} else {
> +		old_pte = *ptep;
>   		if (op == GSTAGE_OP_CLEAR)
>   			set_pte(ptep, __pte(0));
>   		else if (op == GSTAGE_OP_WP)
>   			set_pte(ptep, __pte(pte_val(ptep_get(ptep)) & ~_PAGE_WRITE));
> -		gstage_remote_tlb_flush(kvm, ptep_level, addr);
> +		if (pte_val(*ptep) != pte_val(old_pte))
> +			gstage_remote_tlb_flush(kvm, ptep_level, addr);
>   	}
>   }
>   

Reviewed-by: Atish Patra <atishp@...osinc.com>