linux-kernel - Re: [RFC PATCH 6/8] KVM: x86: Implement kvm_arch_{, pre_}vcpu_map

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Date: Tue, 12 Mar 2024 16:03:40 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Kai Huang <kai.huang@...el.com>
Cc: "dmatlack@...gle.com" <dmatlack@...gle.com>, 
	"federico.parola@...ito.it" <federico.parola@...ito.it>, 
	"isaku.yamahata@...il.com" <isaku.yamahata@...il.com>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>, 
	"michael.roth@....com" <michael.roth@....com>, Isaku Yamahata <isaku.yamahata@...el.com>, 
	"pbonzini@...hat.com" <pbonzini@...hat.com>
Subject: Re: [RFC PATCH 6/8] KVM: x86: Implement kvm_arch_{, pre_}vcpu_map_memory()

On Tue, Mar 12, 2024, Kai Huang wrote:
> On Tue, 2024-03-12 at 21:41 +0000, Huang, Kai wrote:
> > On Tue, 2024-03-12 at 07:20 -0700, Sean Christopherson wrote:
> > > On Tue, Mar 12, 2024, Kai Huang wrote:
> > > > > Wait. KVM doesn't *need* to do PAGE.ADD from deep in the MMU.  The only inputs to
> > > > > PAGE.ADD are the gfn, pfn, tdr (vm), and source.  The S-EPT structures need to be
> > > > > pre-built, but when they are built is irrelevant, so long as they are in place
> > > > > before PAGE.ADD.
> > > > > 
> > > > > Crazy idea.  For TDX S-EPT, what if KVM_MAP_MEMORY does all of the SEPT.ADD stuff,
> > > > > which doesn't affect the measurement, and even fills in KVM's copy of the leaf EPTE, 
> > > > > but tdx_sept_set_private_spte() doesn't do anything if the TD isn't finalized?
> > > > > 
> > > > > Then KVM provides a dedicated TDX ioctl(), i.e. what is/was KVM_TDX_INIT_MEM_REGION,
> > > > > to do PAGE.ADD.  KVM_TDX_INIT_MEM_REGION wouldn't need to map anything, it would
> > > > > simply need to verify that the pfn from guest_memfd() is the same as what's in
> > > > > the TDP MMU.
> > > > 
> > > > One small question:
> > > > 
> > > > What if the memory region passed to KVM_TDX_INIT_MEM_REGION hasn't been pre-
> > > > populated?  If we want to make KVM_TDX_INIT_MEM_REGION work with these regions,
> > > > then we still need to do the real map.  Or we can make KVM_TDX_INIT_MEM_REGION
> > > > return error when it finds the region hasn't been pre-populated?
> > > 
> > > Return an error.  I don't love the idea of bleeding so many TDX details into
> > > userspace, but I'm pretty sure that ship sailed a long, long time ago.
> > 
> > In this case, IIUC the KVM_MAP_MEMORY ioctl() will be mandatory for TDX
> > (presumbly also SNP) guests, but _optional_ for other VMs.  Not sure whether
> > this is ideal.

No, just TDX.  SNP's RMP purely works with pfns, i.e. the enforcement layer comes
into play *after* the stage-2 page table walks.  KVM can zap NPTs for SNP VMs at
will.

> > And just want to make sure I understand the background correctly:
> > 
> > The KVM_MAP_MEMORY ioctl() is supposed to be generic, and it should be able to
> > be used by any VM but not just CoCo VMs (including SW_PROTECTED ones)?
> > 
> > But it is only supposed to be used by the VMs which use guest_memfd()?  Because
> > IIUC for normal VMs using mmap() we already have MAP_POPULATE for this purpose.
> > 
> > Looking at [*], it doesn't say what kind of VM the sender was trying to use.
> > 
> > Therefore can we interpret KVM_MAP_MEMORY ioctl() is effectively for CoCo VMs? 
> > SW_PROTECTED VMs can also use guest_memfd(), but I believe nobody is going to
> > use it seriously.
> > 
> > [*] https://lore.kernel.org/all/65262e67-7885-971a-896d-ad9c0a760907@polito.it/
> > 
> > 
> 
> Hmm.. Just after sending I realized the MAP_POPULATE only pre-populate page
> table at host side, not EPT...

Yep, exactly.

> So the KVM_MAP_MEMORY is indeed can be used by _ALL_ VMs.  My bad :-(