linux-kernel - Definition of PMD_FLAGS_DEC_WP in arch/x86/mm/mem_encrypt

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Message-ID: <20201108163715.GA206902@rani.riverdale.lan>
Date:   Sun, 8 Nov 2020 11:37:15 -0500
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     Tom Lendacky <thomas.lendacky@....com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Definition of PMD_FLAGS_DEC_WP in arch/x86/mm/mem_encrypt_identity.c

Hi, I have a question about this definition in
arch/x86/mm/mem_encrypt_identity.c:

	#define PMD_FLAGS_LARGE         (__PAGE_KERNEL_LARGE_EXEC & ~_PAGE_GLOBAL)

	#define PMD_FLAGS_DEC           PMD_FLAGS_LARGE
	#define PMD_FLAGS_DEC_WP        ((PMD_FLAGS_DEC & ~_PAGE_CACHE_MASK) | \
					 (_PAGE_PAT | _PAGE_PWT))

_PAGE_CACHE_MASK and _PAGE_PAT are for 4k pages, not 2M pages. The
definition of PMD_FLAGS_DEC_WP clears the PSE bit by masking out
_PAGE_CACHE_MASK, and sets it again by setting _PAGE_PAT, resulting in
PMD_FLAGS_DEC_WP actually being write-through, not write-protected,
using PAT index 1.

Shouldn't the definition be

	#define PMD_FLAGS_DEC_WP	(PMD_FLAGS_DEC | _PAGE_PAT_LARGE | _PAGE_PWT)

for write-protected using PAT index 5?

I guess the difference doesn't actually matter for encrypt-in-place? But
mem_encrypt_boot.S takes pains to initialize PA5 to be write-protected,
and it looks like it won't actually be used.

Thanks.