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

[Andrew Jones <ajones@ventanamicro.com>]

On Wed, May 28, 2025 at 07:16:11AM -0700, Atish Patra wrote:
> 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.

If an SBI spec version provides an extension that can be emulated by
userspace, then userspace could choose to advertise that spec version,
implement a BASE probe function that advertises the extension, and
implement the extension, even if the KVM version running is older
and unaware of it. But, in order to do that, we need KVM to exit to
userspace for all unknown SBI calls and to allow BASE to be overridden
by userspace. The new KVM CAP ioctl allows opting into that new behavior.

The old KVM with new VMM configuration isn't totally far-fetched. While
host kernels tend to get updated regularly to include security fixes,
enterprise kernels tend to stop adding features at some point in order
to maximize stability. While enterprise VMMs would also eventually stop
adding features, enterprise consumers are always free to use their own
VMMs (at their own risk). So, there's a real chance we could have
deployments with older, stable KVM where users want to enable later SBI
extensions, and, in some cases, that should be possible by just updating
the VMM -- but only if KVM is only acting as an SBI implementation
accelerator and not as a userspace SBI implementation gatekeeper.

Thanks,
drew

> 
> > 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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