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

[Isaku Yamahata <isaku.yamahata@intel.com>]

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.
-- 
Isaku Yamahata <isaku.yamahata@intel.com>