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Date:   Thu, 17 Mar 2022 16:58:20 +0300
From:   "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     mingo@redhat.com, bp@alien8.de, dave.hansen@intel.com,
        luto@kernel.org, peterz@infradead.org,
        sathyanarayanan.kuppuswamy@linux.intel.com, aarcange@redhat.com,
        ak@linux.intel.com, dan.j.williams@intel.com, david@redhat.com,
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        joro@8bytes.org, jpoimboe@redhat.com, knsathya@kernel.org,
        pbonzini@redhat.com, sdeep@vmware.com, seanjc@google.com,
        tony.luck@intel.com, vkuznets@redhat.com, wanpengli@tencent.com,
        thomas.lendacky@amd.com, brijesh.singh@amd.com, x86@kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCHv6 05/30] x86/tdx: Exclude shared bit from __PHYSICAL_MASK
Message-ID: <20220317135820.frdppktft4flzhha@black.fi.intel.com>
References: <20220316020856.24435-1-kirill.shutemov@linux.intel.com>
 <20220316020856.24435-6-kirill.shutemov@linux.intel.com>
 <87czil303j.ffs@tglx>
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In-Reply-To: <87czil303j.ffs@tglx>
Precedence: bulk

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