linux-kernel - Re: [RFC v2 14/32] x86/tdx: Handle port I/O

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Message-ID: <CAPcyv4hymb73WZ7QqNsDyKiPzsFdPJF63+MLnOTfJMsQBFvSDg@mail.gmail.com>
Date:   Mon, 10 May 2021 18:07:06 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     "Kuppuswamy, Sathyanarayanan" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>
Cc:     Andi Kleen <ak@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Kirill Shutemov <kirill.shutemov@...ux.intel.com>,
        Kuppuswamy Sathyanarayanan <knsathya@...nel.org>,
        Raj Ashok <ashok.raj@...el.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC v2 14/32] x86/tdx: Handle port I/O

On Mon, May 10, 2021 at 5:30 PM Kuppuswamy, Sathyanarayanan
<sathyanarayanan.kuppuswamy@...ux.intel.com> wrote:
[..]
> It is mainly used by functions like __tdx_hypercall(),__tdx_hypercall_vendor_kvm()
> and tdx_in{b,w,l}.
>
> u64 __tdx_hypercall(u64 fn, u64 r12, u64 r13, u64 r14, u64 r15,
>                      struct tdx_hypercall_output *out);
> u64 __tdx_hypercall_vendor_kvm(u64 fn, u64 r12, u64 r13, u64 r14,
>                                 u64 r15, struct tdx_hypercall_output *out);
>
> struct tdx_hypercall_output {
>          u64 r11;
>          u64 r12;
>          u64 r13;
>          u64 r14;
>          u64 r15;
> };

Why is this by register name and not something like:

struct tdx_hypercall_payload {
  u64 data[5];
};

...because the code in this patch is reading the payload out of a
stack relative offset, not r11.

>
>
> Functions like __tdx_hypercall() and __tdx_hypercall_vendor_kvm() are used
> by TDX guest to request services (like RDMSR, WRMSR,GetQuote, etc) from VMM
> using TDCALL instruction. do_tdx_hypercall() is the helper function (in
> tdcall.S) which actually implements this ABI.
>
> As per current ABI, VMM will use registers R11-R15 to share the output
> values with the guest.

Which ABI, __tdx_hypercall_vendor_kvm()? The code is putting the
payload on the stack, so I'm not sure what ABI you are referring to?


> So we have defined the structure
> struct tdx_hypercall_output to group all output registers and make it easier
> to share it with users of the TDCALLs. This is Linux defined structure.
>
> If there are any changes in TDCALL ABI for VMM, we might have to extend
> this structure to accommodate new output register changes.  So if we
> define TDVMCALL_OUTPUT_SIZE as 40, we will have modify this value for
> any future struct tdx_hypercall_output changes. So to avoid it, we have
> allocated double the size.
>
> May be I should define it as,
>
> #define TDVMCALL_OUTPUT_SIZE            sizeof(struct tdx_hypercall_output)

An arrangement like that seems more reasonable than a seemingly
arbitrary number and an ominous warning about things that may happen
in the future.