linux-kernel - Re: [Xen-devel] [PATCH] xen: switch to post-init routines in xen mmu.c earlier

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Message-ID: <54898AD1.5060006@cantab.net>
Date:	Thu, 11 Dec 2014 12:15:13 +0000
From:	David Vrabel <dvrabel@...tab.net>
To:	Juergen Gross <jgross@...e.com>,
	David Vrabel <david.vrabel@...rix.com>,
	linux-kernel@...r.kernel.org, xen-devel@...ts.xensource.com,
	konrad.wilk@...cle.com, boris.ostrovsky@...cle.com
Subject: Re: [Xen-devel] [PATCH] xen: switch to post-init routines in xen
 mmu.c earlier

On 11/12/14 05:32, Juergen Gross wrote:
> On 12/10/2014 07:07 PM, David Vrabel wrote:
>> On 10/12/14 15:56, Juergen Gross wrote:
>>> With the virtual mapped linear p2m list the post-init mmu operations
>>> must be used for setting up the p2m mappings, as in case of
>>> CONFIG_FLATMEM the init routines may trigger BUGs.
>>>
>>> Reported-by: Boris Ostrovsky <boris.ostrovsky@...cle.com>
>>> Signed-off-by: Juergen Gross <jgross@...e.com>
>>> ---
>>>   arch/x86/xen/mmu.c | 2 +-
>>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/arch/x86/xen/mmu.c b/arch/x86/xen/mmu.c
>>> index 6ab6150..a1a429a 100644
>>> --- a/arch/x86/xen/mmu.c
>>> +++ b/arch/x86/xen/mmu.c
>>> @@ -1225,6 +1225,7 @@ static void __init xen_pagetable_p2m_setup(void)
>>>   static void __init xen_pagetable_init(void)
>>>   {
>>>       paging_init();
>>> +    xen_post_allocator_init();
>>>
>>>       xen_pagetable_p2m_setup();
>>>
>>
>> This feels very chicken-and-egg to me:  To setup the P2M we need to use
>> the MMU ops that use the P2M...
>>
>> Please explain very clearly why this is all safe.
> 
> Okay. paging_init() sets up all infrastructure needed to switch to the
> post-init mmu ops done by xen_post_allocator_init(). With the virtual
> mapped linear p2m list we need some mmu ops during setup of this list,
> so we have to switch to the correct mmu ops as soon as possible.
> 
> The p2m list is usable from the beginning, just expansion requires to
> have established the new linear mapping. So the call of
> xen_remap_memory() had to be introduced, but this is not due to the
> mmu ops requiring this.
> 
> Summing it up: calling xen_post_allocator_init() not directly after
> paging_init() was conceptually wrong in the beginning, it just didn't
> matter up to now as no functions used between the two calls needed
> some critical mmu ops (e.g. alloc_pte). This has changed now, so I
> corrected it.

I've added this to the commit message and applied to
devel/for-linus-3.19.  If the tests pass I will consider sending a
further pull request for 3.19 including the linear p2m changes early
next week.

Thanks.

David
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