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

Tao Su
2024-01-12T01:11:42Z

On Thu, Jan 11, 2024 at 08:25:01AM -0800, Sean Christopherson wrote:
> On Thu, Jan 11, 2024, Tao Su wrote:
> > On Wed, Jan 10, 2024 at 08:26:25AM -0800, Sean Christopherson wrote:
> > > On Wed, Jan 10, 2024, Chao Gao wrote:
> > > > On Tue, Jan 09, 2024 at 04:23:40PM -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.
> > > >
> > > > This assumes procfs is enabled in Kconfig and userspace has permission to
> > > > access /proc/cpuinfo. But it isn't always true. So, I think it is better to
> > > > advertise max addressable GPA via KVM ioctls.
> > >
> > > Hrm, so the help for PROC_FS says:
> > >
> > > Several programs depend on this, so everyone should say Y here.
> > >
> > > Given that this is working around something that is borderline an erratum, I'm
> > > inclined to say that userspace shouldn't simply assume the worst if /proc isn't
> > > available. Practically speaking, I don't think a "real" VM is likely to be
> > > affected; AFAIK, there's no reason for QEMU or any other VMM to _need_ to expose
> > > a memslot at GPA[51:48] unless the VM really has however much memory that is
> > > (hundreds of terabytes?). And a if someone is trying to run such a massive VM on
> > > such a goofy CPU...
> >
> > It is unusual to assign a huge RAM to guest, but passthrough a device also may trigger
> > this issue which we have met, i.e. alloc memslot for the 64bit BAR which can set
> > bits[51:48]. BIOS can control the BAR address, e.g. seabios moved 64bit pci window
> > to end of address space by using advertised physical bits[1].
>
> Drat. Do you know if these CPUs are going to be productized? We'll still need
> something in KVM either way, but whether or not the problems are more or less
> limited to funky software setups might influence how we address this.

Yes, please see the CPU model I submitted[1].

[1] https://lore.kernel.org/all/20231206131923.1192066-1-tao1.su@linux.intel.com/

Thanks,
Tao

>