Re: [PATCH 1/5] x86/shim: map and unmap page tables in replace_va_mapping

[Jan Beulich <jbeulich@xxxxxxxx>]

On 23.03.2020 10:41, Hongyan Xia wrote:
> --- a/xen/arch/x86/pv/shim.c
> +++ b/xen/arch/x86/pv/shim.c
> @@ -169,15 +169,19 @@ static void __init replace_va_mapping(struct domain *d, 
> l4_pgentry_t *l4start,
>                                        unsigned long va, mfn_t mfn)
>  {
>      l4_pgentry_t *pl4e = l4start + l4_table_offset(va);
> -    l3_pgentry_t *pl3e = l4e_to_l3e(*pl4e) + l3_table_offset(va);
> -    l2_pgentry_t *pl2e = l3e_to_l2e(*pl3e) + l2_table_offset(va);
> -    l1_pgentry_t *pl1e = l2e_to_l1e(*pl2e) + l1_table_offset(va);
> +    l3_pgentry_t *pl3e = map_l3t_from_l4e(*pl4e) + l3_table_offset(va);
> +    l2_pgentry_t *pl2e = map_l2t_from_l3e(*pl3e) + l2_table_offset(va);
> +    l1_pgentry_t *pl1e = map_l1t_from_l2e(*pl2e) + l1_table_offset(va);
>      struct page_info *page = mfn_to_page(l1e_get_mfn(*pl1e));
>  
>      put_page_and_type(page);
>  
>      *pl1e = l1e_from_mfn(mfn, (!is_pv_32bit_domain(d) ? L1_PROT
>                                                        : COMPAT_L1_PROT));
> +
> +    UNMAP_DOMAIN_PAGE(pl1e);
> +    UNMAP_DOMAIN_PAGE(pl2e);
> +    UNMAP_DOMAIN_PAGE(pl3e);
>  }

I disagree to an approach like this: Why have three pending mappings
when one at a time will do? Map-read/write-unmap three times is what
you want here, even if this is more code churn.

Jan