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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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.

~Andrew