Re: [PATCH] x86/cpu: Add a VMX flag to enumerate 5-level EPT support to userspace

Sean Christopherson
2024-02-23T01:37:00Z

On Tue, 09 Jan 2024 16:23:40 -0800, Sean Christopherson wrote:
> Add a VMX flag in /proc/cpuinfo, ept_5level, so that userspace can query
> whether or not the CPU supports 5-level EPT paging. EPT capabilities are
> enumerated via MSR, i.e. aren't accessible to userspace without help from
> the kernel, and knowing whether or not 5-level EPT is supported is sadly
> necessary for userspace to correctly configure KVM VMs.
>
> When EPT is enabled, bits 51:49 of guest physical addresses are consumed
> if and only if 5-level EPT is enabled. For CPUs with MAXPHYADDR > 48, KVM
> *can't* map all legal guest memory if 5-level EPT is unsupported, e.g.
> creating a VM with RAM (or anything that gets stuffed into KVM's memslots)
> above bit 48 will be completely broken.
>
> [...]

Applied to kvm-x86 vmx, with a massaged changelog to avoid presenting this as a
bug fix (and finally fixed the 51:49=>51:48 goof):

Add a VMX flag in /proc/cpuinfo, ept_5level, so that userspace can query
whether or not the CPU supports 5-level EPT paging. EPT capabilities are
enumerated via MSR, i.e. aren't accessible to userspace without help from
the kernel, and knowing whether or not 5-level EPT is supported is useful
for debug, triage, testing, etc.

For example, when EPT is enabled, bits 51:48 of guest physical addresses
are consumed by the CPU if and only if 5-level EPT is enabled. For CPUs
with MAXPHYADDR > 48, KVM *can't* map all legal guest memory if 5-level
EPT is unsupported, making it more or less necessary to know whether or
not 5-level EPT is supported.

[1/1] x86/cpu: Add a VMX flag to enumerate 5-level EPT support to userspace
https://github.com/kvm-x86/linux/commit/b1a3c366cbc7

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