linux-kernel - Re: [PATCH 4/5] RISC-V: KVM: Enable envcfg and sstateen bits lazily

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Message-Id: <D9U3YFOPMSEF.15BJIA8CET3RT@ventanamicro.com>
Date: Mon, 12 May 2025 12:25:36 +0200
From: Radim Krčmář <rkrcmar@...tanamicro.com>
To: "Atish Patra" <atish.patra@...ux.dev>, "Anup Patel"
 <anup@...infault.org>, "Atish Patra" <atishp@...shpatra.org>, "Paul
 Walmsley" <paul.walmsley@...ive.com>, "Palmer Dabbelt"
 <palmer@...belt.com>, "Alexandre Ghiti" <alex@...ti.fr>
Cc: <kvm@...r.kernel.org>, <kvm-riscv@...ts.infradead.org>,
 <linux-riscv@...ts.infradead.org>, <linux-kernel@...r.kernel.org>,
 "linux-riscv" <linux-riscv-bounces@...ts.infradead.org>
Subject: Re: [PATCH 4/5] RISC-V: KVM: Enable envcfg and sstateen bits lazily

2025-05-09T15:38:55-07:00, Atish Patra <atish.patra@...ux.dev>:
> On 5/8/25 6:32 AM, Radim Krčmář wrote:
>> 2025-05-05T14:39:29-07:00, Atish Patra <atishp@...osinc.com>:
>>> SENVCFG and SSTATEEN CSRs are controlled by HSENVCFG(62) and
>>> SSTATEEN0(63) bits in hstateen. Enable them lazily at runtime
>>> instead of bootime.
>>>
>>> Signed-off-by: Atish Patra <atishp@...osinc.com>
>>> ---
>>> diff --git a/arch/riscv/kvm/vcpu_insn.c b/arch/riscv/kvm/vcpu_insn.c
>>> @@ -256,9 +256,37 @@ int kvm_riscv_vcpu_hstateen_lazy_enable(struct kvm_vcpu *vcpu, unsigned int csr_
>>>   	return KVM_INSN_CONTINUE_SAME_SEPC;
>>>   }
>>>   
>>> +static int kvm_riscv_vcpu_hstateen_enable_senvcfg(struct kvm_vcpu *vcpu,
>>> +						  unsigned int csr_num,
>>> +						  unsigned long *val,
>>> +						  unsigned long new_val,
>>> +						  unsigned long wr_mask)
>>> +{
>>> +	return kvm_riscv_vcpu_hstateen_lazy_enable(vcpu, csr_num, SMSTATEEN0_HSENVCFG);
>>> +}
>> Basically the same comments as for [1/5]:
>>
>> Why don't we want to set the ENVCFG bit (62) unconditionally?
>>
>> It would save us the trap on first access.  We don't get anything from
>> the trap, so it looks like a net negative to me.
>
> We want to lazy enablement is to make sure that hypervisor is aware of 
> the what features
> guest is using. We don't want to necessarily enable the architecture 
> states for the guest if guest doesn't need it.
>
> We need lazy enablement for CTR like features anyways. This will align 
> all the the features controlled
> by stateen in the same manner. The cost is just a single trap at the 
> boot time.
>
> IMO, it's fair trade off.

Yeah, as long as we are doing something with the information from the
trap.