linux-kernel - Re: [PATCH] x86_64: disable identity mappings statically

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Date:	Sat, 12 Feb 2011 09:57:26 +0100
From:	Jiri Olsa <jolsa@...hat.com>
To:	Brian Gerst <brgerst@...il.com>
Cc:	x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86_64: disable identity mappings statically

On Fri, Feb 11, 2011 at 04:59:31PM -0500, Brian Gerst wrote:
> On Fri, Feb 11, 2011 at 3:40 PM, Jiri Olsa <jolsa@...hat.com> wrote:
> > On Fri, Feb 11, 2011 at 03:19:11PM -0500, Brian Gerst wrote:
> >> On Fri, Feb 11, 2011 at 2:13 PM, Jiri Olsa <jolsa@...hat.com> wrote:
> >> > On Fri, Feb 11, 2011 at 12:59:47PM -0500, Brian Gerst wrote:
> >> >> On Fri, Feb 11, 2011 at 12:07 PM, Jiri Olsa <jolsa@...hat.com> wrote:
> >> >> > On Fri, Feb 11, 2011 at 08:46:41AM -0800, Eric W. Biederman wrote:
> >> >> >> Jiri Olsa <jolsa@...hat.com> writes:
> >> >> >>
> >> >> >> > hi,
> >> >> >> >
> >> >> >> > while browsing the page table setup code, I noticed the x86_64 head
> >> >> >> > code might not need the identity mappings at all.
> >> >> >> > It seems it's ok to switch it off completely from the begining,
> >> >> >> > unless I'm missing something.
> >> >> >>
> >> >> >> Have you tested it?
> >> >> >
> >> >> > yes, I booted it with no problem
> >> >>
> >> >> The only reason this doesn't crash is because the identity mappings
> >> >> provided by the boot code are marked as global, and therefore might
> >> >> not be flushed by simply loading cr3.  The cpu can evict TLB entries
> >> >> at any time though, so it's a bad idea to run without the identity
> >> >> mappings even for the brief moment before jumping to the virtual
> >> >> address.
> >> >
> >> > I added code for flushing whole TLB (including global pages) and it
> >> > still boots (attached).
> >> >
> >> > I'm sorry if I'm missing something obvious (probably the TLB flushing
> >> > code is wrong), but I'd like to understand this part.
> >> >
> >> > What instruction/action would require the identity mapping,
> >> > after the page table is set?
> >> >
> >> > thanks (and again sry for noise :) )
> >> > jirka
> >> >
> >> >
> >> > ---
> >> > diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
> >> > index c55e6fa..073f489 100644
> >> > --- a/arch/x86/kernel/head_64.S
> >> > +++ b/arch/x86/kernel/head_64.S
> >> > @@ -165,6 +165,13 @@ ENTRY(secondary_startup_64)
> >> >        movl    $(X86_CR4_PAE | X86_CR4_PGE), %eax
> >> >        movq    %rax, %cr4
> >> >
> >> > +       /* invalidate whole TLB */
> >> > +       movq %cr4, %rax
> >> > +       movq %rax, %rdx
> >> > +       andq $~X86_CR4_PGE, %rax
> >> > +       movq %rax, %cr4
> >> > +       movq %rdx, %cr4
> >> > +
> >> >        /* Setup early boot stage 4 level pagetables. */
> >> >        movq    $(init_level4_pgt - __START_KERNEL_map), %rax
> >> >        addq    phys_base(%rip), %rax
> >> >
> >>
> >> The way you have it, it will immediately reload the global identity
> >> entry into the TLB when it executes the next instruction, because cr3
> >> is still pointing to the old pagetables.  Disable PGE during or
> >> immediately after the load of cr3 to make sure the global identity
> >> entries are flushed.
> >
> > you're right, when I put it after setting cr3 it crashed
> > but I still don't understand what instruction took it down..?
> 
> The instruction immediately following the flush (doesn't matter what
> it is).  You flushed the entry for the page you were executing from,
> so the cpu has to reload the entry for that page so that it can fetch
> the next instruction.  Since that page doesn't exist in the new page
> table, it crashes.

ops, rip is the one I missed... cool :)

thanks for explanation,
jirka

> 
> Getting back to the original patch.  Here is the relevant text from
> the Intel System Programming Guide:
> ------------------------------
> 4.10.4.1 Operations that Invalidate TLBs and Paging-Structure Caches
> ...
> The processor is always free to invalidate additional entries in the
> TLBs and paging-structure
> caches. The following are some examples:
> ...
> MOV to CR3 may invalidate TLB entries for global pages.
> ------------------------------
> 
> So even if it just so happens to work on your particular cpu, it is
> not guaranteed to always work.
> 
> --
> Brian Gerst
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