Re: [PATCH for-4.18] x86/mem_sharing: add missing m2p entry when mapping shared_info page

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 20/10/2023 2:14 am, Henry Wang wrote:
> Hi all,
>
>> On Oct 20, 2023, at 07:48, Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:
>>
>> On 18/10/2023 9:02 am, Tamas K Lengyel wrote:
>>> When mapping in the shared_info page to a fork the m2p entry wasn't set
>>> resulting in the shared_info being reset even when the fork reset was called
>>> with only reset_state and not reset_memory. This results in an extra
>>> unnecessary TLB flush.
>>>
>>> Fixes: 1a0000ac775 ("mem_sharing: map shared_info page to same gfn during 
>>> fork")
>>> Signed-off-by: Tamas K Lengyel <tamas@xxxxxxxxxxxxx>
>>> ---
>>> xen/arch/x86/mm/mem_sharing.c | 2 ++
>>> 1 file changed, 2 insertions(+)
>>>
>>> diff --git a/xen/arch/x86/mm/mem_sharing.c b/xen/arch/x86/mm/mem_sharing.c
>>> index 94b6b782ef..142258f16a 100644
>>> --- a/xen/arch/x86/mm/mem_sharing.c
>>> +++ b/xen/arch/x86/mm/mem_sharing.c
>>> @@ -1847,6 +1847,8 @@ static int copy_special_pages(struct domain *cd, 
>>> struct domain *d)
>>>                                 p2m_ram_rw, p2m->default_access, -1);
>>>             if ( rc )
>>>                 return rc;
>>> +
>>> +            set_gpfn_from_mfn(mfn_x(new_mfn), gfn_x(old_gfn));
>>>         }
>>>     }
>>>
>> Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>>
>> CC Henry.  This needs a view about a release ack.
>>
>> Cons: it's been broken since Xen 4.14 and we're very deep into the 4.18
>> code freeze.
>>
>> Pros: it's a bug and would clearly qualify for backport, and is in a
>> niche feature so isn't plausibly going to adversely affect other users.
> Given the fact that it will be backported anyway, I had the same discussion 
> with Juergen
> in his thread [1]. So if we can bear the risk of delaying merging this patch 
> for a week,
> would it be ok to wait for the release and backport this patch to the stable 
> tree? Thanks!
>
> [1] 
> https://lore.kernel.org/xen-devel/83E6DCF6-926C-43A6-94D2-EB3B2C444309@xxxxxxx/
>
> Kind regards,
> Henry

That's fine.  I'll pull this into my next branch for when the 4.19 tree
opens.

~Andrew