Re: [PATCH v18 00/18] KVM RISC-V Support

[Greg Kroah-Hartman]

On Wed, May 19, 2021 at 07:21:54AM +0200, Greg Kroah-Hartman wrote:
> On Wed, May 19, 2021 at 10:40:13AM +0530, Anup Patel wrote:
> > On Wed, May 19, 2021 at 10:28 AM Greg Kroah-Hartman
> > <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
> > >
> > > On Wed, May 19, 2021 at 09:05:35AM +0530, Anup Patel wrote:
> > > > From: Anup Patel <anup@xxxxxxxxxxxxxx>
> > > >
> > > > This series adds initial KVM RISC-V support. Currently, we are able to boot
> > > > Linux on RV64/RV32 Guest with multiple VCPUs.
> > > >
> > > > Key aspects of KVM RISC-V added by this series are:
> > > > 1. No RISC-V specific KVM IOCTL
> > > > 2. Minimal possible KVM world-switch which touches only GPRs and few CSRs
> > > > 3. Both RV64 and RV32 host supported
> > > > 4. Full Guest/VM switch is done via vcpu_get/vcpu_put infrastructure
> > > > 5. KVM ONE_REG interface for VCPU register access from user-space
> > > > 6. PLIC emulation is done in user-space
> > > > 7. Timer and IPI emuation is done in-kernel
> > > > 8. Both Sv39x4 and Sv48x4 supported for RV64 host
> > > > 9. MMU notifiers supported
> > > > 10. Generic dirtylog supported
> > > > 11. FP lazy save/restore supported
> > > > 12. SBI v0.1 emulation for KVM Guest available
> > > > 13. Forward unhandled SBI calls to KVM userspace
> > > > 14. Hugepage support for Guest/VM
> > > > 15. IOEVENTFD support for Vhost
> > > >
> > > > Here's a brief TODO list which we will work upon after this series:
> > > > 1. SBI v0.2 emulation in-kernel
> > > > 2. SBI v0.2 hart state management emulation in-kernel
> > > > 3. In-kernel PLIC emulation
> > > > 4. ..... and more .....
> > > >
> > > > This series can be found in riscv_kvm_v18 branch at:
> > > > https//github.com/avpatel/linux.git
> > > >
> > > > Our work-in-progress KVMTOOL RISC-V port can be found in riscv_v7 branch
> > > > at: https//github.com/avpatel/kvmtool.git
> > > >
> > > > The QEMU RISC-V hypervisor emulation is done by Alistair and is available
> > > > in master branch at: https://git.qemu.org/git/qemu.git
> > > >
> > > > To play around with KVM RISC-V, refer KVM RISC-V wiki at:
> > > > https://github.com/kvm-riscv/howto/wiki
> > > > https://github.com/kvm-riscv/howto/wiki/KVM-RISCV64-on-QEMU
> > > > https://github.com/kvm-riscv/howto/wiki/KVM-RISCV64-on-Spike
> > > >
> > > > Changes since v17:
> > > >  - Rebased on Linux-5.13-rc2
> > > >  - Moved to new KVM MMU notifier APIs
> > > >  - Removed redundant kvm_arch_vcpu_uninit()
> > > >  - Moved KVM RISC-V sources to drivers/staging for compliance with
> > > >    Linux RISC-V patch acceptance policy
> > >
> > > What is this new "patch acceptance policy" and what does it have to do
> > > with drivers/staging?
> > 
> > The Linux RISC-V patch acceptance policy is here:
> > Documentation/riscv/patch-acceptance.rst
> > 
> > As-per this policy, the Linux RISC-V maintainers will only accept
> > patches for frozen/ratified RISC-V extensions. Basically, it links the
> > Linux RISC-V development process with the RISC-V foundation
> > process which is painfully slow.
> > 
> > The KVM RISC-V patches have been sitting on the lists for almost
> > 2 years now. The requirements for freezing RISC-V H-extension
> > (hypervisor extension) keeps changing and we are not clear when
> > it will be frozen. In fact, quite a few people have already implemented
> > RISC-V H-extension in hardware as well and KVM RISC-V works
> > on real HW as well.
> > 
> > Rationale of moving KVM RISC-V to drivers/staging is to continue
> > KVM RISC-V development without breaking the Linux RISC-V patch
> > acceptance policy until RISC-V H-extension is frozen. Once, RISC-V
> > H-extension is frozen we will move KVM RISC-V back to arch/riscv
> > (like other architectures).
> 
> Wait, no, this has nothing to do with what drivers/staging/ is for and
> how it is used.  Again, not ok.
> 
> > > What does drivers/staging/ have to do with this at all?  Did anyone ask
> > > the staging maintainer about this?
> > 
> > Yes, Paolo (KVM maintainer) suggested having KVM RISC-V under
> > drivers/staging until RISC-V H-extension is frozen and continue the
> > KVM RISC-V development from there.
> 
> staging is not for stuff like this at all.  It is for code that is
> self-contained (not this) and needs work to get merged into the main
> part of the kernel (listed in a TODO file, and is not this).
> 
> It is not a dumping ground for stuff that arch maintainers can not seem
> to agree on, and it is not a place where you can just randomly play
> around with user/kernel apis with no consequences.
> 
> So no, sorry, not going to take this code at all.

And to be a bit more clear about this, having other subsystem
maintainers drop their unwanted code on this subsystem, _without_ even
asking me first is just not very nice.  All of a sudden I am now
responsible for this stuff, without me even being asked about it.
Should I start throwing random drivers into the kvm subsystem for them
to maintain because I don't want to?  :)

If there's really no other way to do this, than to put it in staging,
let's talk about it.  But saying "this must go here" is not a
conversation...

thanks,

greg k-h