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

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Date:   Thu, 17 Mar 2022 15:39:08 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
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 16:58, Kirill A. Shutemov wrote:

> 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?

Yes.