linux-kernel - Re: [PATCH 1/2] x86-32: Fix crashes with CPU hotplug on AMD machines

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Date:	Wed, 04 Aug 2010 16:05:47 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Borislav Petkov <bp@...64.org>
CC:	mingo@...e.hu, tglx@...utronix.de, andreas.herrmann3@....com,
	conny.seidel@....com, joerg.roedel@....com, Bhavna.Sarathy@....com,
	greg@...ah.com, x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] x86-32: Fix crashes with CPU hotplug on AMD machines

On 08/04/2010 09:45 AM, Borislav Petkov wrote:
> 
>         2. Do not use swapper_pg_dir to boot secondary CPUs like 64-bit
>         does.
> 
> This patch implements solution 2. It introduces a trampoline_pg_dir
> which has the same layout as swapper_pg_dir with low_mappings. This page
> table is used as the initial page table of the booting CPU. Later in the
> bringup process, it switches to swapper_pg_dir and does a global TLB
> flush. This fixes the crashes in our test cases.
> 

I would like to keep around a page directory with the low mappings
around -- and not use it for kernel threads -- at all times *anyway*.
This means we can remove any current hacks that we have to do around S3
entry and exit, for example.

--- a/arch/x86/kernel/head_32.S
+++ b/arch/x86/kernel/head_32.S
@@ -328,7 +328,7 @@ ENTRY(startup_32_smp)
 /*
  * Enable paging
  */
-	movl $pa(swapper_pg_dir),%eax
+	movl pa(initial_page_table), %eax
 	movl %eax,%cr3		/* set the page table pointer.. */
 	movl %cr0,%eax
 	orl  $X86_CR0_PG,%eax
@@ -608,6 +608,8 @@ ignore_int:
 .align 4
 ENTRY(initial_code)
 	.long i386_start_kernel
+ENTRY(initial_page_table)
+	.long pa(swapper_pg_dir)

 /*
  * BSS section
@@ -623,6 +625,10 @@ ENTRY(swapper_pg_dir)
 #endif
 swapper_pg_fixmap:
 	.fill 1024,4,0
+#ifdef CONFIG_X86_TRAMPOLINE
+ENTRY(trampoline_pg_dir)
+	.fill 1024,4,0
+#endif

I don't really see why this makes sense, though.  It would make more
sense that the initial page table we set up becomes trampoline_pg_dir;
we can then set up and change to swapper_pg_dir almost immediately.

I realize this isn't how the 64-bit code works at the moment, but in a
lot of ways I think it would be better if it did.

	-hpa
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