linux-kernel - Re: [PATCH 10/16] KVM: x86/tdp_mmu: Support TDX private mapping for TDP MMU

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Date: Tue, 21 May 2024 09:15:20 -0700
From: Isaku Yamahata <isaku.yamahata@...el.com>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
Cc: "Yamahata, Isaku" <isaku.yamahata@...el.com>,
	"dmatlack@...gle.com" <dmatlack@...gle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"seanjc@...gle.com" <seanjc@...gle.com>,
	"Huang, Kai" <kai.huang@...el.com>,
	"sagis@...gle.com" <sagis@...gle.com>,
	"isaku.yamahata@...ux.intel.com" <isaku.yamahata@...ux.intel.com>,
	"Aktas, Erdem" <erdemaktas@...gle.com>,
	"Zhao, Yan Y" <yan.y.zhao@...el.com>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"pbonzini@...hat.com" <pbonzini@...hat.com>,
	"isaku.yamahata@...il.com" <isaku.yamahata@...il.com>
Subject: Re: [PATCH 10/16] KVM: x86/tdp_mmu: Support TDX private mapping for
 TDP MMU

On Tue, May 21, 2024 at 03:07:50PM +0000,
"Edgecombe, Rick P" <rick.p.edgecombe@...el.com> wrote:

> > 1.4.2 Guest Physical Address Translation
> >   Transition to SEAM VMX non-root operation is formatted to require Extended
> >   Page Tables (EPT) to be enabled. In SEAM VMX non-root operation, there
> > should
> >   be two EPTs active: the private EPT specified using the EPTP field of the
> > VMCS
> >   and a shared EPT specified using the Shared-EPTP field of the VMCS.
> >   When translating a GPA using the shared EPT, an EPT misconfiguration can
> > occur
> >   if the entry is present and the physical address bits in the range
> >   (MAXPHYADDR-1) to (MAXPHYADDR-TDX_RESERVED_KEYID_BITS) are set, i.e., if
> >   configured with a TDX private KeyID.
> >   If the CPU's maximum physical-address width (MAXPA) is 52 and the guest
> >   physical address width is configured to be 48, accesses with GPA bits 51:48
> >   not all being 0 can cause an EPT-violation, where such EPT-violations are
> > not
> >   mutated to #VE, even if the “EPT-violations #VE” execution control is 1.
> >   If the CPU's physical-address width (MAXPA) is less than 48 and the SHARED
> > bit
> >   is configured to be in bit position 47, GPA bit 47 would be reserved, and
> > GPA
> >   bits 46:MAXPA would be reserved. On such CPUs, setting bits 51:48 or bits
> >   46:MAXPA in any paging structure can cause a reserved bit page fault on
> >   access.
> 
> In "if the entry is present and the physical address bits in the range
> (MAXPHYADDR-1) to (MAXPHYADDR-TDX_RESERVED_KEYID_BITS) are set", it's not clear
> to be if "physical address bits" is referring to the GPA or the "entry" (meaning
> the host pfn). The "entry" would be my guess.
> 
> It is also confusing when it talks about "guest physical address". It must mean
> 4 vs 5 level paging? How else is the shared EPT walker supposed to know the
> guest maxpa. In which case it would be consistent with normal EPT behavior. But
> the assertions around reserved bit page faults are surprising.
> 
> Based on those guesses, I'm not sure the below code is correct. We wouldn't need
> to remove keyid bits from the GFN.
> 
> Maybe we should clarify the spec? Or are you confident reading it the other way?

I'll read them more closely. At least the following patch is broken.
-- 
Isaku Yamahata <isaku.yamahata@...el.com>