Re: [PATCH v2 1/5] x86/shim: map and unmap page tables in replace_va_mapping

[Jan Beulich <jbeulich@xxxxxxxx>]

On 08.04.2020 15:36, Hongyan Xia wrote:
> --- a/xen/arch/x86/pv/shim.c
> +++ b/xen/arch/x86/pv/shim.c
> @@ -168,16 +168,17 @@ const struct platform_bad_page *__init 
> pv_shim_reserved_pages(unsigned int *size
>  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);
> +    l4_pgentry_t l4e = l4start[l4_table_offset(va)];
> +    l3_pgentry_t l3e = l3e_from_l4e(l4e, l3_table_offset(va));
> +    l2_pgentry_t l2e = l2e_from_l3e(l3e, l2_table_offset(va));
> +    l1_pgentry_t *pl1e = map_l1t_from_l2e(l2e) + 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);
>  }

As said before, here and below I think it should be unmap_domain_page().

> --- a/xen/include/asm-x86/page.h
> +++ b/xen/include/asm-x86/page.h
> @@ -196,6 +196,19 @@ static inline l4_pgentry_t l4e_from_paddr(paddr_t pa, 
> unsigned int flags)
>  #define map_l2t_from_l3e(x)        (l2_pgentry_t 
> *)map_domain_page(l3e_get_mfn(x))
>  #define map_l3t_from_l4e(x)        (l3_pgentry_t 
> *)map_domain_page(l4e_get_mfn(x))
>  
> +/* Unlike lYe_to_lXe(), lXe_from_lYe() do not rely on the direct map. */
> +#define l2e_from_l3e(l3e, offset) ({                        \
> +        const l2_pgentry_t *l2t = map_l2t_from_l3e(l3e);    \
> +        l2_pgentry_t l2e = l2t[offset];                     \
> +        UNMAP_DOMAIN_PAGE(l2t);                             \
> +        l2e; })
> +
> +#define l3e_from_l4e(l4e, offset) ({                        \
> +        const l3_pgentry_t *l3t = map_l3t_from_l4e(l4e);    \
> +        l3_pgentry_t l3e = l3t[offset];                     \
> +        UNMAP_DOMAIN_PAGE(l3t);                             \
> +        l3e; })

I think l1e_from_l2e() should be introduced at the same time, even
if for now it's unused. I also think, like we do elsewhere, that
macro-local variables would better have _ suffixes, to avoid
possible variable aliasing issues.

Jan