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Message-ID: <f3911e98aa56060012b9cf28c0cf904e895decf7.camel@intel.com>
Date: Thu, 13 Jul 2023 09:20:03 +0000
From: "Huang, Kai" <kai.huang@...el.com>
To: "peterz@...radead.org" <peterz@...radead.org>
CC: "Hansen, Dave" <dave.hansen@...el.com>,
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Subject: Re: [PATCH 09/10] x86/virt/tdx: Wire up basic SEAMCALL functions
On Thu, 2023-07-13 at 11:03 +0200, Peter Zijlstra wrote:
> On Thu, Jul 13, 2023 at 08:18:09AM +0000, Huang, Kai wrote:
> > On Thu, 2023-07-13 at 09:42 +0200, Peter Zijlstra wrote:
> > > On Thu, Jul 13, 2023 at 03:46:52AM +0000, Huang, Kai 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.
> > >
> > > I suspect the kvm_vcpu_arch::regs layout is given by hardware; so the
> > > only option would be to make tdx_module_args match that. It's a slightly
> > > unfortunate layout, but meh.
> > >
> > > Then you can simply do:
> > >
> > > __seamcall_saved_ret(leaf, (struct tdx_module_args *)vcpu->arch->regs);
> > >
> > >
> >
> > I don't think the layout matches hardware, especially I think there's no
> > "hardware layout" for GPRs that are concerned here. They are just for KVM
> > itself to save guest's registers when the guest exits to KVM, so that KVM can
> > restore them when returning back to the guest.
>
> Either way around it should be possible to make them match I suppose.
> Ideally we get the callee-clobbered regs first, but if not, I don't
> think that's too big of a problem.
>
Yeah we can. I'll leave this to KVM TDX patches.
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