Re: [ANNOUNCE] PUCK Notes - 2024.04.03 - TDX Upstreaming Strategy

Huang, Kai
2024-04-10T14:03:30Z

On Tue, 2024-04-09 at 18:12 -0700, Isaku Yamahata wrote:
> On Mon, Apr 08, 2024 at 06:51:40PM +0000,
> Sean Christopherson <seanjc@google.com> wrote:
>
> > On Mon, Apr 08, 2024, Edgecombe, Rick P wrote:
> > > On Mon, 2024-04-08 at 09:20 -0700, Sean Christopherson wrote:
> > > > > Another option is that, KVM doesn't allow userspace to configure
> > > > > CPUID(0x8000_0008).EAX[7:0]. Instead, it provides a gpaw field in struct
> > > > > kvm_tdx_init_vm for userspace to configure directly.
> > > > >
> > > > > What do you prefer?
> > > >
> > > > Hmm, neither.  I think the best approach is to build on Gerd's series to have KVM
> > > > select 4-level vs. 5-level based on the enumerated guest.MAXPHYADDR, not on
> > > > host.MAXPHYADDR.
> > >
> > > So then GPAW would be coded to basically best fit the supported guest.MAXPHYADDR within KVM. QEMU
> > > could look at the supported guest.MAXPHYADDR and use matching logic to determine GPAW.
> >
> > Off topic, any chance I can bribe/convince you to wrap your email replies closer
> > to 80 chars, not 100? Yeah, checkpath no longer complains when code exceeds 80
> > chars, but my brain is so well trained for 80 that it actually slows me down a
> > bit when reading mails that are wrapped at 100 chars.
> >
> > > Or are you suggesting that KVM should look at the value of CPUID(0X8000_0008).eax[23:16] passed from
> > > userspace?
> >
> > This. Note, my pseudo-patch incorrectly looked at bits 15:8, that was just me
> > trying to go off memory.
> >
> > > I'm not following the code examples involving struct kvm_vcpu. Since TDX
> > > configures these at a VM level, there isn't a vcpu.
> >
> > Ah, I take it GPAW is a VM-scope knob? I forget where we ended up with the ordering
> > of TDX commands vs. creating vCPUs. Does KVM allow creating vCPU structures in
> > advance of the TDX INIT call? If so, the least awful solution might be to use
> > vCPU0's CPUID.
>
> The current order is, KVM vm creation (KVM_CREATE_VM),
> KVM vcpu creation(KVM_CREATE_VCPU), TDX VM initialization (KVM_TDX_INIT_VM).
> and TDX VCPU initialization(KVM_TDX_INIT_VCPU).
> We can call KVM_SET_CPUID2 before KVM_TDX_INIT_VM. We can remove cpuid part
> from struct kvm_tdx_init_vm by vcpu0 cpuid.

What's the reason to call KVM_TDX_INIT_VM after KVM_CREATE_VCPU?

I guess I have been away for this for too long time, but I had believed
KVM_TDX_INIT_VM is called before creating any vcpu, which turns out to be wrong.

I am not against to make KVM_TDX_INIT_VM must be called after creating (at least
one?) vcpu if there's good reason, but it seems if the purpose is just to pass
CPUID(0x8000_0008).EAX[23:16] to KVM so KVM can determine GPAW for TDX guest,
then we can also make KVM_TDX_INIT_VM to pass that.

KVM just need to manually handle CPUID(0x8000_0008) in KVM_TDX_INIT_VM, but
that's the thing KVM needs to do anyway even if we use vcpu0's CPUID.

Am I missing anything?