linux-kernel - Re: [PATCH v3 2/2] RISC-V KVM: Remove unnecessary HGATP csr_read

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Message-ID: <aKXeXd4pZvoQmYB9@troy-wujie14pro-arch>
Date: Wed, 20 Aug 2025 22:40:29 +0800
From: Troy Mitchell <troy.mitchell@...ux.dev>
To: fangyu.yu@...ux.alibaba.com, anup@...infault.org, atish.patra@...ux.dev,
	paul.walmsley@...ive.com, palmer@...belt.com, aou@...s.berkeley.edu,
	alex@...ti.fr
Cc: guoren@...nel.org, kvm@...r.kernel.org, kvm-riscv@...ts.infradead.org,
	linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Troy Mitchell <troy.mitchell@...ux.dev>
Subject: Re: [PATCH v3 2/2] RISC-V KVM: Remove unnecessary HGATP csr_read

On Wed, Aug 20, 2025 at 08:59:52PM +0800, fangyu.yu@...ux.alibaba.com wrote:
> From: "Guo Ren (Alibaba DAMO Academy)" <guoren@...nel.org>
> 
> The HGATP has been set to zero in gstage_mode_detect(), so there
> is no need to save the old context. Unify the code convention
> with gstage_mode_detect().
> 
> Signed-off-by: Guo Ren (Alibaba DAMO Academy) <guoren@...nel.org>
> Signed-off-by: Fangyu Yu <fangyu.yu@...ux.alibaba.com>
> ---
>  arch/riscv/kvm/vmid.c | 5 +----
>  1 file changed, 1 insertion(+), 4 deletions(-)
> 
> diff --git a/arch/riscv/kvm/vmid.c b/arch/riscv/kvm/vmid.c
> index 5f33625f4070..abb1c2bf2542 100644
> --- a/arch/riscv/kvm/vmid.c
> +++ b/arch/riscv/kvm/vmid.c
> @@ -25,15 +25,12 @@ static DEFINE_SPINLOCK(vmid_lock);
>  
>  void __init kvm_riscv_gstage_vmid_detect(void)
>  {
> -	unsigned long old;
> -
>  	/* Figure-out number of VMID bits in HW */
> -	old = csr_read(CSR_HGATP);
>  	csr_write(CSR_HGATP, (kvm_riscv_gstage_mode << HGATP_MODE_SHIFT) | HGATP_VMID);
>  	vmid_bits = csr_read(CSR_HGATP);
>  	vmid_bits = (vmid_bits & HGATP_VMID) >> HGATP_VMID_SHIFT;
>  	vmid_bits = fls_long(vmid_bits);
> -	csr_write(CSR_HGATP, old);
> +	csr_write(CSR_HGATP, 0);
Is setting HGATP to 0 in gstage_mode_detect meaningless now?
If so, it might be better to drop it and just keep the one here.

                - Troy

>  
>  	/* We polluted local TLB so flush all guest TLB */
>  	kvm_riscv_local_hfence_gvma_all();
> -- 
> 2.49.0
> 
> 
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> linux-riscv mailing list
> linux-riscv@...ts.infradead.org
> http://lists.infradead.org/mailman/listinfo/linux-riscv

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