On Thu, May 29, 2025 at 11:44:38AM -0700, Atish Patra wrote: > > On 5/29/25 3:24 AM, Radim Krčmář wrote: > > I originally gave up on the idea, but I feel kinda bad for Drew now, so > > trying again: > > I am sorry if some of my replies came across in the wrong way. That was > never > the intention. Not at all. Radim only meant that I was defending his patches, even though he wasn't :-) > > > > 2025-05-28T12:21:59-07:00, Atish Patra <atish.patra@xxxxxxxxx>: > > > On 5/28/25 8:09 AM, Andrew Jones wrote: > > > > 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@xxxxxxxxx>: > > > > > > > > On 5/23/25 6:31 AM, Radim Krčmář wrote: > > > > > > > > > 2025-05-22T12:03:43-07:00, Atish Patra <atishp@xxxxxxxxxxxx>: > > > > > > > > > > Upgrade the SBI version to v3.0 so that corresponding features > > > > > > > > > > can be enabled in the guest. > > > > > > > > > > > > > > > > > > > > Signed-off-by: Atish Patra <atishp@xxxxxxxxxxxx> > > > > > > > > > > --- > > > > > > > > > > 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 > > > You mean only the version field in BASE - Correct ? > > No, "BASE probe function" is the sbi_probe_extension() ecall. > > > > > > by userspace. The new KVM CAP ioctl allows opting into that new behavior. > > > But why we need a new IOCTL for that ? We can achieve that with existing > > > one reg interface with improvements. > > It's an existing IOCTL with a new data payload, but I can easily use > > ONE_REG if you want to do everything through that. > > > > KVM doesn't really need any other IOCTL than ONE_REGs, it's just > > sometimes more reasonable to use a different IOCTL, like ENABLE_CAP. > > > > > > 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 > > > I think we are years away from that happening (if it happens). My > > > suggestion was not to > > > try to build a world where no body lives ;). When we get to that > > > scenario, the default KVM > > > shipped will have many extension implemented. So there won't be much > > > advantage to > > > reimplement them in the user space. We can also take an informed > > > decision at that time > > > if the current selective forwarding approach is better > > Please don't repeat the design of SUSP/SRST/DBCN. > > Seeing them is one of the reasons why I proposed the new interface. > > > > "Blindly" forwarding DBCN to userspace is even a minor optimization. :) > > > > > or we need to > > > blindly forward any > > > unknown SBI calls to the user space. > > Yes, KVM has to do what userpace configures it to do. > > > > I don't think that implementing unsupported SBI extensions in KVM is > > important -- they should not be a hot path. > > > > > > 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. > > > But some of the SBI extensions are so fundamental that it must be > > > implemented in KVM > > > for various reasons pointed by Anup on other thread. > > No, SBI does not have to be implemented in KVM at all. > > > > We do have a deep disagreement on what is virtualization and the role of > > KVM in it. I think that userspace wants a generic ISA accelerator. > > I think the disagreement is the role of SBI in KVM virtualization rather > than > a generic virtualization and the role of KVM in it. I completely agree that > KVM should act as an accelerator and defer the control to the user space in > most of the cases > such e.g I/O operations or system related functionalities. However, SBI > specification solves > much wider problems than those. Broadly we can categorize SBI > functionalities into the following > areas > > 1. Bridging ISA GAP > 2. Higher Privilege Assistance > 3. Virtualization > 4. Platform abstraction > 5. Confidential computing > > For #1, #3 and #5, I believe user space shouldn't be involved in > implementation > some of them are in hot path as well. IMO, userspace should still be in control of whether or not it's involved in #1, #3, and #5. It may make little sense for it to be involved, but the choice should still be its. > For #4 and #2, there are some > opportunities which > can be implemented in user space depending on the exact need. I am still not > clear what is the exact > motivation /right now/ to pursue such a path. May be I missed something. > As per my understanding from our discussion threads, there are two use cases > possible > > 1. userspace wants to update more states in HSM. What are the states user > space should care about scounteren (fixed already in usptream) ? > 2. VMM vs KVM version difference - this may be true in the future depending > on the speed of RISC-V virtualization adoption in the industry. > But we are definitely not there yet. Please let me know if I misunderstood > any use cases. That's what I'm aware of as well, but I see giving userspace back full control of what gets accelerated by KVM, and what doesn't, as a fix, which is why I wouldn't want to delay it any longer. > > > Even if userspace wants SBI for the M-mode interface, security minded > This is probably a 3rd one ? Why we want M-mode interface in the user space > ? > > userspace aims for as little kernel code as possible. > > We trust VMM code more than KVM code ? We should be skeptical of both, which is why we'd rather put as much code in userspace as possible. Insecure/faulty userspace will hopefully have exploits/bugs contained to the single process. An insecure/faulty KVM means the host is compromised/crashed. On x86, Google put a lot of effort into moving instruction emulation out of KVM for security concerns[1]. In general, if it's not a hot path and there's a way to do it in userspace, then it should be done in userspace (or at least there should be an option to use userspace -- each use case can choose what's best for itself). [1] https://www.linux-kvm.org/images/3/3d/01x02-Steve_Rutherford-Performant_Security_Hardening_of_KVM.pdf Thanks, drew > > > Userspace might want to accelerate some SBI extension in KVM, but it > > should not be KVM who decides what userspace wants.
