On Thu, Jul 13, 2023 at 11:44:34AM -0700, Isaku Yamahata wrote: > On Thu, Jul 13, 2023 at 03:46:52AM +0000, > "Huang, Kai" <kai.huang@xxxxxxxxx> wrote: > > > On Wed, 2023-07-12 at 15:15 -0700, Isaku Yamahata wrote: > > > > The SEAMCALL ABI is very similar to the TDCALL ABI and leverages much > > > > TDCALL infrastructure. Wire up basic functions to make SEAMCALLs for > > > > the basic TDX support: __seamcall(), __seamcall_ret() and > > > > __seamcall_saved_ret() which is for TDH.VP.ENTER leaf function. > > > > > > Hi. __seamcall_saved_ret() uses struct tdx_module_arg as input and output. For > > > KVM TDH.VP.ENTER case, those arguments are already in unsigned long > > > kvm_vcpu_arch::regs[]. It's silly to move those values twice. From > > > kvm_vcpu_arch::regs to tdx_module_args. From tdx_module_args to real registers. > > > > > > If TDH.VP.ENTER is the only user of __seamcall_saved_ret(), can we make it to > > > take unsigned long kvm_vcpu_argh::regs[NR_VCPU_REGS]? Maybe I can make the > > > change with TDX KVM patch series. > > > > The assembly code assumes the second argument is a pointer to 'struct > > tdx_module_args'. I don't know how can we change __seamcall_saved_ret() to > > achieve what you said. We might change the kvm_vcpu_argh::regs[NR_VCPU_REGS] to > > match 'struct tdx_module_args''s layout and manually convert part of "regs" to > > the structure and pass to __seamcall_saved_ret(), but it's too hacky I suppose. > > > > This was one concern that I mentioned VP.ENTER can be implemented by KVM in its > > own assembly in the TDX host v12 discussion. I kinda agree we should leverage > > KVM's existing kvm_vcpu_arch::regs[NR_CPU_REGS] infrastructure to minimize the > > code change to the KVM's common infrastructure. If so, I guess we have to carry > > this memory copy burden between two structures. > > > > Btw, I do find KVM's VP.ENTER code is a little bit redundant to the common > > SEAMCALL assembly, which is a good reason for KVM to use __seamcall() variants > > for TDH.VP.ENTER. > > > > So it's a tradeoff I think. > > > > On the other hand, given CoCo VMs normally don't expose all GPRs to VMM, it's > > also debatable whether we should invent another infrastructure to the KVM code > > to handle register access of CoCo VMs too, e.g., we can catch bugs easily when > > KVM tries to access the registers that it shouldn't access. > > Yes, we'd like to save/restore GPRs only for TDVMCALL. Otherwise skip > save/restore. And another case to save/restore GPRs: supports DEBUG TD, which is type of TD guest allows VMM to change its register context, for debugging purpose. > > -- > Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
