linux-kernel - Re: [PATCHv6 05/30] x86/tdx: Exclude shared bit from __PHYSICAL

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Date:   Thu, 17 Mar 2022 16:58:20 +0300
From:   "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     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, seanjc@...gle.com,
        tony.luck@...el.com, vkuznets@...hat.com, wanpengli@...cent.com,
        thomas.lendacky@....com, brijesh.singh@....com, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCHv6 05/30] x86/tdx: Exclude shared bit from __PHYSICAL_MASK

On Thu, Mar 17, 2022 at 01:16:00AM +0100, Thomas Gleixner wrote:
> On Wed, Mar 16 2022 at 05:08, Kirill A. Shutemov wrote:
> > @@ -82,6 +82,14 @@ void __init tdx_early_init(void)
> >  
> >  	cc_set_vendor(CC_VENDOR_INTEL);
> >  
> > +	/*
> > +	 * All bits above GPA width are reserved and kernel treats shared bit
> > +	 * as flag, not as part of physical address.
> > +	 *
> > +	 * Adjust physical mask to only cover valid GPA bits.
> > +	 */
> > +	physical_mask &= GENMASK_ULL(gpa_width - 2, 0);
> > +
> 
> Hrm. I forgot about the second use case for gpa_width, but my comment
> about ordering still stands. OTOH:
> 
>          GENMASK_ULL(gpa_width - 2, 0) == BIT_UL(gpa_width - 1) - 1
> 
> right? So you really can consolidate on the fact that cc_mask is a
> single bit which is above the guests physical address space boundary.
> 
> I.e. make the code tell the story instead of adding lengthy comments
> explaining the obfuscation.

So it will looks something like this:


	cc_set_vendor(CC_VENDOR_INTEL);
	cc_mask = get_cc_mask();
	cc_set_mask(cc_mask);

	/*
	 * All bits above GPA width are reserved and kernel treats shared bit
	 * as flag, not as part of physical address.
	 *
	 * Adjust physical mask to only cover valid GPA bits.
	 */
	physical_mask &= cc_mask - 1;

I still think these comments are useful. I hided comment for cc_mask
calclulation inside get_cc_mask().

Does it look fine to you?

-- 
 Kirill A. Shutemov