linux-kernel - Re: [PATCH v4 1/6] x86/cpu: Use a new feature flag for 5 level paging

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Message-ID: <20250519121541.GAaCsg7YZ3-HDYgiBu@fat_crate.local>
Date: Mon, 19 May 2025 14:15:41 +0200
From: Borislav Petkov <bp@...en8.de>
To: Ard Biesheuvel <ardb@...nel.org>
Cc: Ard Biesheuvel <ardb+git@...gle.com>, linux-kernel@...r.kernel.org,
	x86@...nel.org, Ingo Molnar <mingo@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Brian Gerst <brgerst@...il.com>,
	"Kirill A. Shutemov" <kirill@...temov.name>
Subject: Re: [PATCH v4 1/6] x86/cpu: Use a new feature flag for 5 level paging

On Mon, May 19, 2025 at 11:46:34AM +0200, Ard Biesheuvel wrote:
> That is what the old code does. It results in the flag transiently
> being set and cleared again, which is what I am trying to avoid.

Right, something like this clumsy thing ontop. It'll have to be macro-ized
properly and we had macros for those somewhere - need to grep...

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 104944e93902..a6a1892a9215 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -704,7 +704,10 @@ static const char *table_lookup_model(struct cpuinfo_x86 *c)
 }
 
 /* Aligned to unsigned long to avoid split lock in atomic bitmap ops */
-__u32 cpu_caps_cleared[NCAPINTS + NBUGINTS] __aligned(sizeof(unsigned long));
+__u32 cpu_caps_cleared[NCAPINTS + NBUGINTS] __aligned(sizeof(unsigned long)) = {
+	[X86_FEATURE_LA57 / 32] = BIT(X86_FEATURE_LA57 & 0x1f)
+};
+
 __u32 cpu_caps_set[NCAPINTS + NBUGINTS] __aligned(sizeof(unsigned long));
 
 #ifdef CONFIG_X86_32

asm looks correct to me:

        .type   cpu_caps_cleared, @object
        .size   cpu_caps_cleared, 96    
cpu_caps_cleared:
        .zero   64
        .long   65536
        .zero   28

-- 
Regards/Gruss,
    Boris.

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