linux-kernel - Re: [PATCHv3 07/17] x86/mm: Preserve KeyID on pte_modify() and pgprot

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Date:   Fri, 15 Jun 2018 08:31:57 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Kai Huang <kai.huang@...ux.intel.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCHv3 07/17] x86/mm: Preserve KeyID on pte_modify() and
 pgprot_modify()

On 06/15/2018 08:27 AM, Kirill A. Shutemov wrote:
> On Fri, Jun 15, 2018 at 01:43:03PM +0000, Dave Hansen wrote:
>> On 06/15/2018 05:57 AM, Kirill A. Shutemov wrote:
>>>>> +#define _PAGE_CHG_MASK	(PTE_PFN_MASK_MAX | _PAGE_PCD | _PAGE_PWT |		\
>>>>>  			 _PAGE_SPECIAL | _PAGE_ACCESSED | _PAGE_DIRTY |	\
>>>>>  			 _PAGE_SOFT_DIRTY)
>>>>>  #define _HPAGE_CHG_MASK (_PAGE_CHG_MASK | _PAGE_PSE)
>>>> This makes me a bit nervous.  We have some places (here) where we
>>>> pretend that the KeyID is part of the paddr and then other places like
>>>> pte_pfn() where it's not.
>>> Other option is to include KeyID mask into _PAGE_CHG_MASK. But it means
>>> _PAGE_CHG_MASK would need to reference *two* variables: physical_mask and
>>> mktme_keyid_mask. I mentioned this in the commit message.
>>
>> Why can't it be one variable with a different name that's populated by
>> OR'ing physical_mask and mktme_keyid_mask together?
> 
> My point is that we don't need variables at all here.
> 
> Architecture defines range of bits in PTE used for PFN. MKTME reduces the
> number of bits for PFN. PTE_PFN_MASK_MAX represents the original
> architectural range, before MKTME stole these bits.
> 
> PTE_PFN_MASK_MAX is constant -- on x86-64 bits 51:12 -- regardless of
> MKTME support.

Then please just rename the make PTE_<SOMETHING>_MASK where <SOMETHING>
includes both the concept of a physical address and a MKTME keyID.  Just
don't call it a pfn if it is not used in physical addressing.

>> Whatever you come up with will probably fine, as long as things that are
>> called "PFN" or physical address don't also get used for keyID bits.
> 
> We are arguing about macros used exactly once. Is it really so confusing?

Yes.