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. -- Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
