Re: [PATCH v10 01/13] x86/mm: rewrite virt_to_xen_l*e

[Jan Beulich <jbeulich@xxxxxxxx>]

On 21.04.2021 16:15, Hongyan Xia wrote:
> From: Wei Liu <wei.liu2@xxxxxxxxxx>
> 
> Rewrite those functions to use the new APIs. Modify its callers to unmap
> the pointer returned. Since alloc_xen_pagetable_new() is almost never
> useful unless accompanied by page clearing and a mapping, introduce a
> helper alloc_map_clear_xen_pt() for this sequence.
> 
> Signed-off-by: Wei Liu <wei.liu2@xxxxxxxxxx>
> Signed-off-by: Hongyan Xia <hongyxia@xxxxxxxxxx>

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
albeit ...

> @@ -4941,33 +4961,33 @@ static l3_pgentry_t *virt_to_xen_l3e(unsigned long v)
>      if ( !(l4e_get_flags(*pl4e) & _PAGE_PRESENT) )
>      {
>          bool locking = system_state > SYS_STATE_boot;
> -        l3_pgentry_t *l3t = alloc_xen_pagetable();
> +        mfn_t l3mfn;
> +        l3_pgentry_t *l3t = alloc_mapped_pagetable(&l3mfn);
>  
>          if ( !l3t )
>              return NULL;
> -        clear_page(l3t);
> +        UNMAP_DOMAIN_PAGE(l3t);

... this immediate unmapping (and then re-mapping below) will imo
want re-doing down the road as well. Even if it's not a severe
performance hit, it's simply odd, at least to me.

Jan