linux-kernel - Re: [RFC v1 22/26] x86/tdx: Exclude Shared bit from __PHYSICAL

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Date:   Tue, 6 Apr 2021 09:12:48 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Kirill Shutemov <kirill.shutemov@...ux.intel.com>,
        Kuppuswamy Sathyanarayanan <knsathya@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Raj Ashok <ashok.raj@...el.com>,
        Sean Christopherson <seanjc@...gle.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC v1 22/26] x86/tdx: Exclude Shared bit from __PHYSICAL_MASK

On 4/6/21 8:54 AM, Kirill A. Shutemov wrote:
> On Thu, Apr 01, 2021 at 01:13:16PM -0700, Dave Hansen wrote:
>>> @@ -56,6 +61,9 @@ static void tdx_get_info(void)
>>>  
>>>  	td_info.gpa_width = rcx & GENMASK(5, 0);
>>>  	td_info.attributes = rdx;
>>> +
>>> +	/* Exclude Shared bit from the __PHYSICAL_MASK */
>>> +	physical_mask &= ~tdx_shared_mask();
>>>  }
>> I wish we had all of these 'physical_mask' manipulations in a single
>> spot.  Can we consolidate these instead of having TDX and SME poke at
>> them individually?
> SME has to do it very early -- from __startup_64() -- as it sets the bit
> on all memory, except what used for communication. TDX can postpone as we
> don't need any shared mapping in very early boot.
> 
> Basically, to make it done from the same place we would need to move TDX
> enumeration earlier into boot. It's risky: everything is more fragile
> there.
> 
> I would rather keep it as is. We should be fine as long as we only allow
> to clear bits from the mask.

I'll buy that.  Could you mention it in the changelog, please?