linux-kernel - Re: [PATCH v6 14/14] RISC-V: KVM: add support for SBI_FWFT_MISALIGNED

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Message-ID: <87b2eade-acda-428e-81af-d4927e517ebe@linux.dev>
Date: Fri, 9 May 2025 11:09:18 -0700
From: Atish Patra <atish.patra@...ux.dev>
To: Clément Léger <cleger@...osinc.com>,
 Paul Walmsley <paul.walmsley@...ive.com>, Palmer Dabbelt
 <palmer@...belt.com>, Anup Patel <anup@...infault.org>,
 Atish Patra <atishp@...shpatra.org>, Shuah Khan <shuah@...nel.org>,
 Jonathan Corbet <corbet@....net>, linux-riscv@...ts.infradead.org,
 linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
 kvm@...r.kernel.org, kvm-riscv@...ts.infradead.org,
 linux-kselftest@...r.kernel.org
Cc: Samuel Holland <samuel.holland@...ive.com>,
 Andrew Jones <ajones@...tanamicro.com>, Deepak Gupta <debug@...osinc.com>
Subject: Re: [PATCH v6 14/14] RISC-V: KVM: add support for
 SBI_FWFT_MISALIGNED_DELEG

On 4/24/25 10:32 AM, ClÃ©ment LÃ©ger wrote:
> SBI_FWFT_MISALIGNED_DELEG needs hedeleg to be modified to delegate
> misaligned load/store exceptions. Save and restore it during CPU
> load/put.
> 
> Signed-off-by: Clément Léger <cleger@...osinc.com>
> Reviewed-by: Deepak Gupta <debug@...osinc.com>
> Reviewed-by: Andrew Jones <ajones@...tanamicro.com>
> ---
>   arch/riscv/kvm/vcpu.c          |  3 +++
>   arch/riscv/kvm/vcpu_sbi_fwft.c | 36 ++++++++++++++++++++++++++++++++++
>   2 files changed, 39 insertions(+)
> 
> diff --git a/arch/riscv/kvm/vcpu.c b/arch/riscv/kvm/vcpu.c
> index 542747e2c7f5..d98e379945c3 100644
> --- a/arch/riscv/kvm/vcpu.c
> +++ b/arch/riscv/kvm/vcpu.c
> @@ -646,6 +646,7 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
>   {
>   	void *nsh;
>   	struct kvm_vcpu_csr *csr = &vcpu->arch.guest_csr;
> +	struct kvm_vcpu_config *cfg = &vcpu->arch.cfg;
>   
>   	vcpu->cpu = -1;
>   
> @@ -671,6 +672,7 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
>   		csr->vstval = nacl_csr_read(nsh, CSR_VSTVAL);
>   		csr->hvip = nacl_csr_read(nsh, CSR_HVIP);
>   		csr->vsatp = nacl_csr_read(nsh, CSR_VSATP);
> +		cfg->hedeleg = nacl_csr_read(nsh, CSR_HEDELEG);
>   	} else {
>   		csr->vsstatus = csr_read(CSR_VSSTATUS);
>   		csr->vsie = csr_read(CSR_VSIE);
> @@ -681,6 +683,7 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
>   		csr->vstval = csr_read(CSR_VSTVAL);
>   		csr->hvip = csr_read(CSR_HVIP);
>   		csr->vsatp = csr_read(CSR_VSATP);
> +		cfg->hedeleg = csr_read(CSR_HEDELEG);

Can we avoid saving hedeleg in vcpu_put path by updating the 
cfg->hedeleg in kvm_sbi_fwft_set_misaligned_delegation.

We already update the hedeleg in vcpu_load path from cfg->hedeleg.
If the next vcpu did not enable delegation it will get the correct 
config written to hedeleg.

This will save us a csr read cost in each VM exit path for something 
that is probably configured once in guest life time.

>   	}
>   }
>   
> diff --git a/arch/riscv/kvm/vcpu_sbi_fwft.c b/arch/riscv/kvm/vcpu_sbi_fwft.c
> index b0f66c7bf010..d16ee477042f 100644
> --- a/arch/riscv/kvm/vcpu_sbi_fwft.c
> +++ b/arch/riscv/kvm/vcpu_sbi_fwft.c
> @@ -14,6 +14,8 @@
>   #include <asm/kvm_vcpu_sbi.h>
>   #include <asm/kvm_vcpu_sbi_fwft.h>
>   
> +#define MIS_DELEG (BIT_ULL(EXC_LOAD_MISALIGNED) | BIT_ULL(EXC_STORE_MISALIGNED))
> +
>   struct kvm_sbi_fwft_feature {
>   	/**
>   	 * @id: Feature ID
> @@ -68,7 +70,41 @@ static bool kvm_fwft_is_defined_feature(enum sbi_fwft_feature_t feature)
>   	return false;
>   }
>   
> +static bool kvm_sbi_fwft_misaligned_delegation_supported(struct kvm_vcpu *vcpu)
> +{
> +	return misaligned_traps_can_delegate();
> +}
> +
> +static long kvm_sbi_fwft_set_misaligned_delegation(struct kvm_vcpu *vcpu,
> +					struct kvm_sbi_fwft_config *conf,
> +					unsigned long value)
> +{
> +	if (value == 1)
> +		csr_set(CSR_HEDELEG, MIS_DELEG);
> +	else if (value == 0)
> +		csr_clear(CSR_HEDELEG, MIS_DELEG);
> +	else
> +		return SBI_ERR_INVALID_PARAM;
> +
> +	return SBI_SUCCESS;
> +}
> +
> +static long kvm_sbi_fwft_get_misaligned_delegation(struct kvm_vcpu *vcpu,
> +					struct kvm_sbi_fwft_config *conf,
> +					unsigned long *value)
> +{
> +	*value = (csr_read(CSR_HEDELEG) & MIS_DELEG) == MIS_DELEG;
> +
> +	return SBI_SUCCESS;
> +}
> +
>   static const struct kvm_sbi_fwft_feature features[] = {
> +	{
> +		.id = SBI_FWFT_MISALIGNED_EXC_DELEG,
> +		.supported = kvm_sbi_fwft_misaligned_delegation_supported,
> +		.set = kvm_sbi_fwft_set_misaligned_delegation,
> +		.get = kvm_sbi_fwft_get_misaligned_delegation,
> +	},
>   };
>   
>   static struct kvm_sbi_fwft_config *