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From: Jim Mattson <jmattson@google.com>
Date: Thu, 11 Jan 2024 13:12:24 -0800
Message-ID: <CALMp9eQzZkGsg2aUeK0vy--qOGCOXpHByGrNK4SFU38jYQZT9A@mail.gmail.com>
Subject: Re: [PATCH] x86/cpu: Add a VMX flag to enumerate 5-level EPT support
 to userspace
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Sean Christopherson <seanjc@google.com>, Tao Su <tao1.su@linux.intel.com>, 
	Chao Gao <chao.gao@intel.com>, Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>, 
	Borislav Petkov <bp@alien8.de>, Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org, 
	kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Yi Lai <yi1.lai@intel.com>, 
	Xudong Hao <xudong.hao@intel.com>
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On Thu, Jan 11, 2024 at 12:02=E2=80=AFPM Paolo Bonzini <pbonzini@redhat.com=
> wrote:
>
> On Thu, Jan 11, 2024 at 5:25=E2=80=AFPM Sean Christopherson <seanjc@googl=
e.com> wrote:
> > > 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 wh=
ich can set
> > > bits[51:48]. BIOS can control the BAR address, e.g. seabios moved 64b=
it 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 st=
ill need
> > something in KVM either way, but whether or not the problems are more o=
r less
> > limited to funky software setups might influence how we address this.
>
> Wait, we do have an API for guest physical address size. It's
> KVM_GET_SUPPORTED_CPUID2: the # of bits is in leaf 0x80000008, bits
> 0:7 of EAX. In fact that leaf is what firmware uses to place the BARs.
> So it just needs to be adjusted for VMX in __do_cpuid_func, and looked
> up in selftests.

We've discussed this. The only *supported* value for guest.MAXPHYADDR
is host.MAXPHYADDR.

If EPT doesn't also support that value, then the only *supported*
configuration is shadow paging.

If someone wants to run with scissors, that's fine, but don't abuse
KVM_GET_SUPPORTED_CPUID2 to return an *unsupported* configuration.