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Re: [Xen-devel] [PATCH] xen/x86: clear per cpu stub page information in cpu_smpboot_free()
- To: Jürgen Groß <jgross@xxxxxxxx>, Wei Liu <wl@xxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Wed, 8 Jan 2020 12:20:08 +0000
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- Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx, Jan Beulich <jbeulich@xxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Delivery-date: Wed, 08 Jan 2020 12:20:24 +0000
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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On 08/01/2020 12:18, Jürgen Groß wrote:
> On 08.01.20 13:16, Wei Liu wrote:
>> On Wed, Jan 08, 2020 at 12:01:48PM +0100, Juergen Gross wrote:
>>> cpu_smpboot_free() removes the stubs for the cpu going offline, but it
>>> isn't clearing the related percpu variables. This will result in
>>> crashes when a stub page is released due to all related cpus gone
>>> offline and one of those cpus going online later.
>>>
>>> Fix that by clearing stubs.addr and stubs.mfn in order to allocate a
>>> new stub page when needed.
>>>
>>> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
>>> ---
>>> xen/arch/x86/smpboot.c | 2 ++
>>> 1 file changed, 2 insertions(+)
>>>
>>> diff --git a/xen/arch/x86/smpboot.c b/xen/arch/x86/smpboot.c
>>> index 7e29704080..46c0729214 100644
>>> --- a/xen/arch/x86/smpboot.c
>>> +++ b/xen/arch/x86/smpboot.c
>>> @@ -945,6 +945,8 @@ static void cpu_smpboot_free(unsigned int cpu,
>>> bool remove)
>>> (per_cpu(stubs.addr, cpu) |
>>> ~PAGE_MASK) + 1);
>>> if ( i == STUBS_PER_PAGE )
>>> free_domheap_page(mfn_to_page(mfn));
>>> + per_cpu(stubs.addr, cpu) = 0;
>>> + per_cpu(stubs.mfn, cpu) = 0;
>>
>> Shouldn't the mfn be set to INVALID_MFN instead?
>
> This would require modifying alloc_stub_page():
>
> if ( *mfn )
> pg = mfn_to_page(_mfn(*mfn));
> else
Correct. per-cpu data is initialised to 0, not to a custom default, so
using INVALID_MFN is more complicated.
~Andrew
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