linux-kernel - Re: [PATCHv3 16/17] x86/mm: Handle encrypted memory in page_to_virt() and _

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Date:   Wed, 13 Jun 2018 11:43:08 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Tom Lendacky <thomas.lendacky@....com>
Cc:     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 16/17] x86/mm: Handle encrypted memory in page_to_virt()
 and __pa()

> diff --git a/arch/x86/include/asm/mktme.h b/arch/x86/include/asm/mktme.h
> index efc0d4bb3b35..d6edcabacfc7 100644
> --- a/arch/x86/include/asm/mktme.h
> +++ b/arch/x86/include/asm/mktme.h
> @@ -43,6 +43,9 @@ void mktme_disable(void);
>  void setup_direct_mapping_size(void);
>  int sync_direct_mapping(void);
>  
> +#define page_to_virt(x) \
> +	(__va(PFN_PHYS(page_to_pfn(x))) + page_keyid(x) * direct_mapping_size)

This looks like a super important memory management function being
defined in some obscure Intel-specific feature header.  How does that work?

>  #else
>  #define mktme_keyid_mask	((phys_addr_t)0)
>  #define mktme_nr_keyids		0
> diff --git a/arch/x86/include/asm/page_64.h b/arch/x86/include/asm/page_64.h
> index 53c32af895ab..ffad496aadad 100644
> --- a/arch/x86/include/asm/page_64.h
> +++ b/arch/x86/include/asm/page_64.h
> @@ -23,7 +23,7 @@ static inline unsigned long __phys_addr_nodebug(unsigned long x)
>  	/* use the carry flag to determine if x was < __START_KERNEL_map */
>  	x = y + ((x > y) ? phys_base : (__START_KERNEL_map - PAGE_OFFSET));
>  
> -	return x;
> +	return x % direct_mapping_size;
>  }

What are the performance implications of this patch?