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

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Message-ID: <20250526-224478e15ee50987124a47ac@orel>
Date: Mon, 26 May 2025 13:13:59 +0200
From: Andrew Jones <ajones@...tanamicro.com>
To: Radim Krčmář <rkrcmar@...tanamicro.com>
Cc: Atish Patra <atish.patra@...ux.dev>, Anup Patel <anup@...infault.org>, 
	Will Deacon <will@...nel.org>, Mark Rutland <mark.rutland@....com>, 
	Paul Walmsley <paul.walmsley@...ive.com>, Palmer Dabbelt <palmer@...belt.com>, 
	Mayuresh Chitale <mchitale@...tanamicro.com>, linux-riscv@...ts.infradead.org, 
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org, 
	Palmer Dabbelt <palmer@...osinc.com>, kvm@...r.kernel.org, kvm-riscv@...ts.infradead.org, 
	linux-riscv <linux-riscv-bounces@...ts.infradead.org>
Subject: Re: [PATCH v3 9/9] RISC-V: KVM: Upgrade the supported SBI version to
 3.0

On Mon, May 26, 2025 at 11:00:30AM +0200, Radim Krčmář wrote:
> 2025-05-23T10:16:11-07:00, Atish Patra <atish.patra@...ux.dev>:
> > On 5/23/25 6:31 AM, Radim Krčmář wrote:
> >> 2025-05-22T12:03:43-07:00, Atish Patra <atishp@...osinc.com>:
> >>> Upgrade the SBI version to v3.0 so that corresponding features
> >>> can be enabled in the guest.
> >>>
> >>> Signed-off-by: Atish Patra <atishp@...osinc.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.

This is covered by your recent patch that provides userspace_sbi.
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