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Message-ID: <20230713184434.GH3894444@ls.amr.corp.intel.com>
Date: Thu, 13 Jul 2023 11:44:34 -0700
From: Isaku Yamahata <isaku.yamahata@...il.com>
To: "Huang, Kai" <kai.huang@...el.com>
Cc: "isaku.yamahata@...il.com" <isaku.yamahata@...il.com>,
"Hansen, Dave" <dave.hansen@...el.com>,
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<sathyanarayanan.kuppuswamy@...ux.intel.com>
Subject: Re: [PATCH 09/10] x86/virt/tdx: Wire up basic SEAMCALL functions
On Thu, Jul 13, 2023 at 03:46:52AM +0000,
"Huang, Kai" <kai.huang@...el.com> 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@...il.com>
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