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

[Tamas K Lengyel <tamas@xxxxxxxxxxxxx>]

On Fri, Oct 20, 2023 at 6:57 AM Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:
>
> 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.

Thanks, I agree that backporting is perfectly fine for this one.

Tamas