linux-kernel - Re: [PATCHv3 10/17] x86/mm: Implement prep_encrypted_page() and arch_free

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Date:   Wed, 13 Jun 2018 11:26:10 -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 10/17] x86/mm: Implement prep_encrypted_page() and
 arch_free_page()

On 06/12/2018 07:39 AM, Kirill A. Shutemov wrote:
> prep_encrypted_page() also takes care about zeroing the page. We have to
> do this after KeyID is set for the page.

This is an implementation detail that has gone unmentioned until now but
has impacted at least half a dozen locations in previous patches.  Can
you rectify that, please?


> +void prep_encrypted_page(struct page *page, int order, int keyid, bool zero)
> +{
> +	int i;
> +
> +	/*
> +	 * The hardware/CPU does not enforce coherency between mappings of the
> +	 * same physical page with different KeyIDs or encrypt ion keys.

What are "encrypt ion"s? :)

> +	 * We are responsible for cache management.
> +	 *
> +	 * We flush cache before allocating encrypted page
> +	 */
> +	clflush_cache_range(page_address(page), PAGE_SIZE << order);
> +
> +	for (i = 0; i < (1 << order); i++) {
> +		WARN_ON_ONCE(lookup_page_ext(page)->keyid);

/* All pages coming out of the allocator should have KeyID 0 */

> +		lookup_page_ext(page)->keyid = keyid;
> +		/* Clear the page after the KeyID is set. */
> +		if (zero)
> +			clear_highpage(page);
> +	}
> +}

How expensive is this?

> +void arch_free_page(struct page *page, int order)
> +{
> +	int i;
> 

	/* KeyId-0 pages were not used for MKTME and need no work */

... or something

> +	if (!page_keyid(page))
> +		return;

Is page_keyid() optimized so that all this goes away automatically when
MKTME is compiled out or unsupported?

> +	for (i = 0; i < (1 << order); i++) {
> +		WARN_ON_ONCE(lookup_page_ext(page)->keyid > mktme_nr_keyids);
> +		lookup_page_ext(page)->keyid = 0;
> +	}
> +
> +	clflush_cache_range(page_address(page), PAGE_SIZE << order);
> +}