[v2] Improve KVM_SET_TSC_KHZ handling for CoCo VMs

[PATCH v2 0/2] Improve KVM_SET_TSC_KHZ handling for CoCo VMs

Posted by Kai Huang 2 months, 3 weeks ago

This series follows Sean's suggestions [1][2] to:

 - Reject vCPU scope KVM_SET_TSC_KHZ ioctl for TSC protected vCPU
 - Reject VM scope KVM_SET_TSC_KHZ ioctl when vCPUs have been created

.. in the discussion of SEV-SNP Secure TSC support series.

v1 -> v2:
 - Protect kvm->create_vcpus check using kvm->lock. - Chao.
 - Add documentation update to each patch.  -Nikunj.
 - Collect RB (Xiaoyao, Nikunj).
 - Switched the patch order to make documentation update easier.

 v1: https://lore.kernel.org/kvm/cover.1752038725.git.kai.huang@intel.com/

Hi Xiaoyao, Nikunj, I added your RB anyway, so let me know if you have
concern :-)

This series has been sanity tested with TDX guests using today's Qemu:

 - With this series Qemu can still run TDX guests successfully.
 - With some hack to the Qemu, both VM and vCPU scope KVM_SET_TSC_KHZ
   ioctls failed as expected.

Kai Huang (2):
  KVM: x86: Reject KVM_SET_TSC_KHZ VM ioctl when vCPUs have been created
  KVM: x86: Reject KVM_SET_TSC_KHZ vCPU ioctl for TSC protected guest

 Documentation/virt/kvm/api.rst |  9 ++++++++-
 arch/x86/kvm/x86.c             | 13 ++++++++++---
 2 files changed, 18 insertions(+), 4 deletions(-)


base-commit: 6c7ecd725e503bf2ca69ff52c6cc48bb650b1f11
-- 
2.50.0

Re: [PATCH v2 0/2] Improve KVM_SET_TSC_KHZ handling for CoCo VMs

Posted by Sean Christopherson 2 months, 3 weeks ago

On Mon, 14 Jul 2025 10:20:18 +1200, Kai Huang wrote:
> This series follows Sean's suggestions [1][2] to:
> 
>  - Reject vCPU scope KVM_SET_TSC_KHZ ioctl for TSC protected vCPU
>  - Reject VM scope KVM_SET_TSC_KHZ ioctl when vCPUs have been created
> 
> .. in the discussion of SEV-SNP Secure TSC support series.
> 
> [...]

Applied patch 2 to kvm-x86 fixes, with a tweaked changelog to call out that
TDX support hasn't yet been released, i.e. that there is no established ABI
to break.

Applied patch 1 to kvm-x86 misc, with tweaked documentation to not imply that
userspace "must" invoke the ioctl.  I think this is the last patch I'll throw
into misc for 6.17?  So in theory, if it breaks userspace, I can simply
truncate it from the pull request.

[1/2] KVM: x86: Reject KVM_SET_TSC_KHZ VM ioctl when vCPUs have been created
      https://github.com/kvm-x86/linux/commit/dcbe5a466c12
[2/2] KVM: x86: Reject KVM_SET_TSC_KHZ vCPU ioctl for TSC protected guest
      https://github.com/kvm-x86/linux/commit/e51cf184d90c

--
https://github.com/kvm-x86/linux/tree/next

Re: [PATCH v2 0/2] Improve KVM_SET_TSC_KHZ handling for CoCo VMs

Posted by Huang, Kai 2 months, 3 weeks ago

On Mon, 2025-07-14 at 17:23 -0700, Sean Christopherson wrote:
> On Mon, 14 Jul 2025 10:20:18 +1200, Kai Huang wrote:
> > This series follows Sean's suggestions [1][2] to:
> > 
> >  - Reject vCPU scope KVM_SET_TSC_KHZ ioctl for TSC protected vCPU
> >  - Reject VM scope KVM_SET_TSC_KHZ ioctl when vCPUs have been created
> > 
> > .. in the discussion of SEV-SNP Secure TSC support series.
> > 
> > [...]
> 
> Applied patch 2 to kvm-x86 fixes, with a tweaked changelog to call out that
> TDX support hasn't yet been released, i.e. that there is no established ABI
> to break.
> 
> Applied patch 1 to kvm-x86 misc, with tweaked documentation to not imply that
> userspace "must" invoke the ioctl.  I think this is the last patch I'll throw
> into misc for 6.17?  So in theory, if it breaks userspace, I can simply
> truncate it from the pull request.

Thanks!

> 
> [1/2] KVM: x86: Reject KVM_SET_TSC_KHZ VM ioctl when vCPUs have been created
>       https://github.com/kvm-x86/linux/commit/dcbe5a466c12
> [2/2] KVM: x86: Reject KVM_SET_TSC_KHZ vCPU ioctl for TSC protected guest
>       https://github.com/kvm-x86/linux/commit/e51cf184d90c

Btw, in the second patch it seems you have:

  Fixes; adafea1 ("KVM: x86: Add infrastructure for secure TSC")

Shouldn't we follow the standard format, i.e.,

  Fixes: adafea110600 ("KVM: x86: Add infrastructure for secure TSC")


?

Re: [PATCH v2 0/2] Improve KVM_SET_TSC_KHZ handling for CoCo VMs

Posted by Sean Christopherson 2 months, 3 weeks ago

On Tue, Jul 15, 2025, Kai Huang wrote:
> On Mon, 2025-07-14 at 17:23 -0700, Sean Christopherson wrote:
> > On Mon, 14 Jul 2025 10:20:18 +1200, Kai Huang wrote:
> > > This series follows Sean's suggestions [1][2] to:
> > > 
> > >  - Reject vCPU scope KVM_SET_TSC_KHZ ioctl for TSC protected vCPU
> > >  - Reject VM scope KVM_SET_TSC_KHZ ioctl when vCPUs have been created
> > > 
> > > .. in the discussion of SEV-SNP Secure TSC support series.
> > > 
> > > [...]
> > 
> > Applied patch 2 to kvm-x86 fixes, with a tweaked changelog to call out that
> > TDX support hasn't yet been released, i.e. that there is no established ABI
> > to break.
> > 
> > Applied patch 1 to kvm-x86 misc, with tweaked documentation to not imply that
> > userspace "must" invoke the ioctl.  I think this is the last patch I'll throw
> > into misc for 6.17?  So in theory, if it breaks userspace, I can simply
> > truncate it from the pull request.
> 
> Thanks!
> 
> > 
> > [1/2] KVM: x86: Reject KVM_SET_TSC_KHZ VM ioctl when vCPUs have been created
> >       https://github.com/kvm-x86/linux/commit/dcbe5a466c12
> > [2/2] KVM: x86: Reject KVM_SET_TSC_KHZ vCPU ioctl for TSC protected guest
> >       https://github.com/kvm-x86/linux/commit/e51cf184d90c
> 
> Btw, in the second patch it seems you have:
> 
>   Fixes; adafea1 ("KVM: x86: Add infrastructure for secure TSC")
> 
> Shouldn't we follow the standard format, i.e.,
> 
>   Fixes: adafea110600 ("KVM: x86: Add infrastructure for secure TSC")

Ugh, yes, the semi-colon is just a typo.  New hash:

  https://github.com/kvm-x86/linux/commit/b24bbb534c2d