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

Xiaoyao Li
2024-02-26T01:30:44Z

On 2/23/2024 9:35 AM, Sean Christopherson wrote:
> 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

Do we need a new KVM CAP for this? This decides how to interact with old
kernel without this patch. In that case, no ept_5level in /proc/cpuinfo,
what should we do in the absence of ept_5level? treat it only 4 level
EPT supported?

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