linux-kernel - Re: [PATCHv2 20/29] x86/tdx: Get page shared bit info from the TDX module

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Date:   Mon, 7 Feb 2022 22:27:20 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        mingo@...hat.com, bp@...en8.de, dave.hansen@...el.com,
        luto@...nel.org, peterz@...radead.org,
        sathyanarayanan.kuppuswamy@...ux.intel.com, aarcange@...hat.com,
        ak@...ux.intel.com, dan.j.williams@...el.com, david@...hat.com,
        hpa@...or.com, jgross@...e.com, jmattson@...gle.com,
        joro@...tes.org, jpoimboe@...hat.com, knsathya@...nel.org,
        pbonzini@...hat.com, sdeep@...are.com, tony.luck@...el.com,
        vkuznets@...hat.com, wanpengli@...cent.com, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCHv2 20/29] x86/tdx: Get page shared bit info from the TDX
 module

On Wed, Feb 02, 2022, Thomas Gleixner wrote:
> On Mon, Jan 24 2022 at 18:02, Kirill A. Shutemov wrote:
> > +static void tdx_get_info(void)
> > +{
> > +	struct tdx_module_output out;
> > +	u64 ret;
> > +
> > +	/*
> > +	 * TDINFO TDX module call is used to get the TD execution environment
> > +	 * information like GPA width, number of available vcpus, debug mode
> > +	 * information, etc. More details about the ABI can be found in TDX
> > +	 * Guest-Host-Communication Interface (GHCI), sec 2.4.2 TDCALL
> > +	 * [TDG.VP.INFO].
> > +	 */
> > +	ret = __tdx_module_call(TDX_GET_INFO, 0, 0, 0, 0, &out);
> > +
> > +	/* Non zero return value indicates buggy TDX module, so panic */
> 
> Can you please get rid of these useless comments all over the place. The
> panic() message tells the same story. Please document the non-obvious
> things.

And why isn't there a tdx_module_call() wrapper to panic() on failure?  IIRC,
that's why the asm routines had the double underscore, but that detail appears
to have been lost.  E.g. __tdx_module_call(TDX_GET_VEINFO, ...) in patch 04 should
also panic, but it currently morphs the #VE into a #GP if it can't retrieve the
info, which will lead to weird "#GPs" on things like vanilla MOV instructions if
something does go wrong.  TDX_ACCEPT_PAGE is the only call into the TDX Module
for which failure is not fatal.