linux-kernel - Re: [PATCH 5/6] x86/hyperv: Support hypercalls for TDX guests

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Date:   Mon, 28 Nov 2022 11:11:05 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Dexuan Cui <decui@...rosoft.com>,
        "Michael Kelley (LINUX)" <mikelley@...rosoft.com>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>,
        "arnd@...db.de" <arnd@...db.de>, "bp@...en8.de" <bp@...en8.de>,
        "brijesh.singh@....com" <brijesh.singh@....com>,
        "Williams, Dan J" <dan.j.williams@...el.com>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        "hpa@...or.com" <hpa@...or.com>,
        "jane.chu@...cle.com" <jane.chu@...cle.com>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        KY Srinivasan <kys@...rosoft.com>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "luto@...nel.org" <luto@...nel.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
        "sathyanarayanan.kuppuswamy@...ux.intel.com" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        "seanjc@...gle.com" <seanjc@...gle.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "tony.luck@...el.com" <tony.luck@...el.com>,
        "wei.liu@...nel.org" <wei.liu@...nel.org>,
        "x86@...nel.org" <x86@...nel.org>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 5/6] x86/hyperv: Support hypercalls for TDX guests

On 11/28/22 11:03, Dexuan Cui wrote:
...
> u64 hv_tdx_hypercall(u64 control, u64 param1, u64 param2)
> {
>         struct tdx_hypercall_args args = { };
> 
>         if (!(control & HV_HYPERCALL_FAST_BIT)) {
>                 if (param1)
>                         param1 = cc_mkdec(param1);
> 
>                 if (param2)
>                         param2 = cc_mkdec(param2);
>         }
> 
>         args.r10 = control;
>         args.rdx = param1;
>         args.r8  = param2;
> 
>         (void)__tdx_hypercall(&args, TDX_HCALL_HAS_OUTPUT);
> 
>         return args.r11;
> }

I still think this is problematic.

The cc_mkdec() should be done on the parameters when the code still
*knows* that they are addresses.

How do we know, for instance, that no hypercall using this interface
will *ever* take the 0x0 physical address as an argument?