Re: [PATCH v3 9/9] RISC-V: KVM: Upgrade the supported SBI version to 3.0

From: Atish Patra <atish.patra@linux.dev>
To: "Andrew Jones" <ajones@ventanamicro.com>,
	"Radim Krčmář" <rkrcmar@ventanamicro.com>
Cc: Atish Patra <atish.patra@linux.dev>,
	Anup Patel <anup@brainfault.org>, Will Deacon <will@kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Paul Walmsley <paul.walmsley@sifive.com>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Mayuresh Chitale <mchitale@ventanamicro.com>,
	linux-riscv@lists.infradead.org,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org,
	Palmer Dabbelt <palmer@rivosinc.com>,
	kvm@vger.kernel.org, kvm-riscv@lists.infradead.org,
	linux-riscv <linux-riscv-bounces@lists.infradead.org>
Subject: Re: [PATCH v3 9/9] RISC-V: KVM: Upgrade the supported SBI version to 3.0
Date: Wed, 28 May 2025 07:16:11 -0700	[thread overview]
Message-ID: <ace8be22-3dba-41b0-81f0-bf6d661b4343@linux.dev> (raw)
In-Reply-To: <20250526-224478e15ee50987124a47ac@orel>

On 5/26/25 4:13 AM, Andrew Jones wrote:
> On Mon, May 26, 2025 at 11:00:30AM +0200, Radim Krčmář wrote:
>> 2025-05-23T10:16:11-07:00, Atish Patra <atish.patra@linux.dev>:
>>> On 5/23/25 6:31 AM, Radim Krčmář wrote:
>>>> 2025-05-22T12:03:43-07:00, Atish Patra <atishp@rivosinc.com>:
>>>>> Upgrade the SBI version to v3.0 so that corresponding features
>>>>> can be enabled in the guest.
>>>>>
>>>>> Signed-off-by: Atish Patra <atishp@rivosinc.com>
>>>>> ---
>>>>> diff --git a/arch/riscv/include/asm/kvm_vcpu_sbi.h b/arch/riscv/include/asm/kvm_vcpu_sbi.h
>>>>> -#define KVM_SBI_VERSION_MAJOR 2
>>>>> +#define KVM_SBI_VERSION_MAJOR 3
>>>> I think it's time to add versioning to KVM SBI implementation.
>>>> Userspace should be able to select the desired SBI version and KVM would
>>>> tell the guest that newer features are not supported.
> 
> We need new code for this, but it's a good idea.
> 
>>>
>>> We can achieve that through onereg interface by disabling individual SBI
>>> extensions.
>>> We can extend the existing onereg interface to disable a specific SBI
>>> version directly
>>> instead of individual ones to save those IOCTL as well.
>>
>> Yes, I am all in favor of letting userspace provide all values in the
>> BASE extension.
> 

We already support vendorid/archid/impid through one reg. I think we 
just need to add the SBI version support to that so that user space can 
set it.

> This is covered by your recent patch that provides userspace_sbi.

Why do we need to invent new IOCTL for this ? Once the user space sets 
the SBI version, KVM can enforce it.

> With that, userspace can disable all extensions that aren't
> supported by a given spec version, disable BASE and then provide
> a BASE that advertises the version it wants. The new code is needed
> for extensions that userspace still wants KVM to accelerate, but then
> KVM needs to be informed it should deny all functions not included in
> the selected spec version.
> 
> Thanks,
> drew
> 
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