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

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Message-ID: <ZfBkle1eZFfjPI8l@google.com>
Date: Tue, 12 Mar 2024 07:20:05 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Kai Huang <kai.huang@...el.com>
Cc: Isaku Yamahata <isaku.yamahata@...el.com>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>, 
	"pbonzini@...hat.com" <pbonzini@...hat.com>, 
	"federico.parola@...ito.it" <federico.parola@...ito.it>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, 
	"isaku.yamahata@...il.com" <isaku.yamahata@...il.com>, "michael.roth@....com" <michael.roth@....com>, 
	"dmatlack@...gle.com" <dmatlack@...gle.com>
Subject: Re: [RFC PATCH 6/8] KVM: x86: Implement kvm_arch_{, pre_}vcpu_map_memory()

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.