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

[Tao Su]

On Mon, Feb 26, 2024 at 09:30:33AM +0800, Xiaoyao Li wrote:
> 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?

Maybe also adding flag for 4-level EPT can be an option. If userspace
checks both 4-level and 5-level are not in /proc/cpuinfo, it can regard
the kernel as old.

Thanks,
Tao

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