Re: [PATCH v2 1/2] KVM: nVMX: Disallow access to vmcs12 fields that aren't supported by "hardware"

[Xin Li <xin@zytor.com>]

> On Dec 30, 2025, at 2:02 PM, Sean Christopherson <seanjc@google.com> wrote:
> 
> Disallow access (VMREAD/VMWRITE) to fields that the loaded incarnation of
> KVM doesn't support, e.g. due to lack of hardware support, as a middle
> ground between allowing access to any vmcs12 field defined by KVM (current
> behavior) and gating access based on the userspace-defined vCPU model (the
> most correct, but costly, implementation).
> 
> Disallowing access to unsupported fields helps a tiny bit in terms of
> closing the virtualization hole (see below), but the main motivation is to
> avoid having to weed out unsupported fields when synchronizing between
> vmcs12 and a shadow VMCS.  Because shadow VMCS accesses are done via
> VMREAD and VMWRITE, KVM _must_ filter out unsupported fields (or eat
> VMREAD/VMWRITE failures), and filtering out just shadow VMCS fields is
> about the same amount of effort, and arguably much more confusing.
> 
> As a bonus, this also fixes a KVM-Unit-Test failure bug when running on
> _hardware_ without support for TSC Scaling, which fails with the same
> signature as the bug fixed by commit ba1f82456ba8 ("KVM: nVMX: Dynamically
> compute max VMCS index for vmcs12"):
> 
>  FAIL: VMX_VMCS_ENUM.MAX_INDEX expected: 19, actual: 17
> 
> Dynamically computing the max VMCS index only resolved the issue where KVM
> was hardcoding max index, but for CPUs with TSC Scaling, that was "good
> enough".
> 
> Cc: Xin Li <xin@zytor.com>
> Cc: Chao Gao <chao.gao@intel.com>
> Cc: Yosry Ahmed <yosry.ahmed@linux.dev>
> Link: https://lore.kernel.org/all/20251026201911.505204-22-xin@zytor.com
> Link: https://lore.kernel.org/all/YR2Tf9WPNEzrE7Xg@google.com
> Signed-off-by: Sean Christopherson <seanjc@google.com>

Reviewed-by: Xin Li <xin@zytor.com <mailto:xin@zytor.com>>