linux-kernel - Re: [PATCH v3 2/4] x86/mm/64: Flush global TLB on boot and AP bringup

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Date:   Tue, 26 Oct 2021 11:55:44 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Joerg Roedel <joro@...tes.org>
Cc:     x86@...nel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, hpa@...or.com,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Joerg Roedel <jroedel@...e.de>,
        Mike Rapoport <rppt@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Brijesh Singh <brijesh.singh@....com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 2/4] x86/mm/64: Flush global TLB on boot and AP bringup

On Fri, Oct 01, 2021 at 05:48:15PM +0200, Joerg Roedel wrote:
> +/*
> + * The __flush_tlb_all() function uses all kinds of state which is not
> + * initialized that early and can not be used here. So the helper below is used
> + * to flush global TLB entries.
> + */
> +static void __init early_flush_tlb_global(void)
> +{
> +	unsigned long cr4 = native_read_cr4();
> +
> +	native_write_cr4(cr4 ^ X86_CR4_PGE);
> +	native_write_cr4(cr4);
> +}

Please make sure now and in the future to avoid such duplication - diff
ontop, for a possible way to do this, at the end.

>  asmlinkage __visible void __init x86_64_start_kernel(char * real_mode_data)
>  {
>  	/*
> @@ -478,6 +491,8 @@ asmlinkage __visible void __init x86_64_start_kernel(char * real_mode_data)
>  	/* Kill off the identity-map trampoline */
>  	reset_early_page_tables();
>  
> +	early_flush_tlb_global();
> +
>  	clear_bss();
>  
>  	clear_page(init_top_pgt);
> diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
> index d8b3ebd2bb85..bd4b6ebafdc3 100644
> --- a/arch/x86/kernel/head_64.S
> +++ b/arch/x86/kernel/head_64.S
> @@ -166,9 +166,26 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)
>  	call	sev_verify_cbit
>  	popq	%rsi
>  
> -	/* Switch to new page-table */
> +	/*
> +	 * Switch to new page-table
> +	 *
> +	 * For the boot CPU this switches to early_top_pgt which still has the
> +	 * indentity mappings present. The secondary CPUs will switch to the
> +	 * init_top_pgt here, away from the trampoline_pgd and unmapping the

"... unmap the... "

> +	 * indentity mapped ranges.
> +	 *
> +	 * Do a global TLB flush after the CR3 switch to make sure the TLB
> +	 * entries from the identity mapping are flushed.

Put this comment...

> +	 */
>  	movq	%rax, %cr3
>  
> +	/* Flush global TLB entries - only needed for secondary CPUs */

... here instead of this one.

> +	movq	%cr4, %rcx
> +	movq	%rcx, %rax
> +	xorq	$X86_CR4_PGE, %rcx
> +	movq	%rcx, %cr4
> +	movq	%rax, %cr4
> +
>  	/* Ensure I am executing from virtual addresses */
>  	movq	$1f, %rax
>  	ANNOTATE_RETPOLINE_SAFE
> -- 

diff --git a/arch/x86/include/asm/tlbflush.h b/arch/x86/include/asm/tlbflush.h
index b587a9ee9cb2..98fa0a114074 100644
--- a/arch/x86/include/asm/tlbflush.h
+++ b/arch/x86/include/asm/tlbflush.h
@@ -261,4 +261,9 @@ extern void arch_tlbbatch_flush(struct arch_tlbflush_unmap_batch *batch);
 
 #endif /* !MODULE */
 
+static inline void __native_tlb_flush_global(unsigned long cr4)
+{
+	native_write_cr4(cr4 ^ X86_CR4_PGE);
+	native_write_cr4(cr4);
+}
 #endif /* _ASM_X86_TLBFLUSH_H */
diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
index 0a93b24d7604..75acb6027a87 100644
--- a/arch/x86/kernel/head64.c
+++ b/arch/x86/kernel/head64.c
@@ -462,19 +462,6 @@ static void __init copy_bootdata(char *real_mode_data)
 	sme_unmap_bootdata(real_mode_data);
 }
 
-/*
- * The __flush_tlb_all() function uses all kinds of state which is not
- * initialized that early and can not be used here. So the helper below is used
- * to flush global TLB entries.
- */
-static void __init early_flush_tlb_global(void)
-{
-	unsigned long cr4 = native_read_cr4();
-
-	native_write_cr4(cr4 ^ X86_CR4_PGE);
-	native_write_cr4(cr4);
-}
-
 asmlinkage __visible void __init x86_64_start_kernel(char * real_mode_data)
 {
 	/*
@@ -496,7 +483,7 @@ asmlinkage __visible void __init x86_64_start_kernel(char * real_mode_data)
 	/* Kill off the identity-map trampoline */
 	reset_early_page_tables();
 
-	early_flush_tlb_global();
+	__native_tlb_flush_global(native_read_cr4());
 
 	clear_bss();
 
diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
index 59ba2968af1b..1e6513f95133 100644
--- a/arch/x86/mm/tlb.c
+++ b/arch/x86/mm/tlb.c
@@ -1148,7 +1148,7 @@ void flush_tlb_one_user(unsigned long addr)
  */
 STATIC_NOPV void native_flush_tlb_global(void)
 {
-	unsigned long cr4, flags;
+	unsigned long flags;
 
 	if (static_cpu_has(X86_FEATURE_INVPCID)) {
 		/*
@@ -1168,11 +1168,7 @@ STATIC_NOPV void native_flush_tlb_global(void)
 	 */
 	raw_local_irq_save(flags);
 
-	cr4 = this_cpu_read(cpu_tlbstate.cr4);
-	/* toggle PGE */
-	native_write_cr4(cr4 ^ X86_CR4_PGE);
-	/* write old PGE again and flush TLBs */
-	native_write_cr4(cr4);
+	__native_tlb_flush_global(this_cpu_read(cpu_tlbstate.cr4));
 
 	raw_local_irq_restore(flags);
 }

-- 
Regards/Gruss,
    Boris.

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