Re: [PATCH] x86/mm: do not mark IO regions as Xen heap

[Jan Beulich <jbeulich@xxxxxxxx>]

On 10.09.2020 15:35, Roger Pau Monne wrote:
> arch_init_memory will treat all the gaps on the physical memory map
> between RAM regions as MMIO and use share_xen_page_with_guest in order
> to assign them to dom_io. This has the side effect of setting the Xen
> heap flag on such pages, and thus is_special_page would then return
> true which is an issue in epte_get_entry_emt because such pages will
> be forced to use write-back cache attributes.
> 
> Fix this by introducing a new helper to assign the MMIO regions to
> dom_io without setting the Xen heap flag on the pages, so that
> is_special_page will return false and the pages won't be forced to use
> write-back cache attributes.
> 
> Fixes: 81fd0d3ca4b2cd ('x86/hvm: simplify 'mmio_direct' check in 
> epte_get_entry_emt()')
> Suggested-by: Jan Beulich <jbeulich@xxxxxxxx>
> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
albeit I'm inclined to add, while committing, a comment ...

> --- a/xen/arch/x86/mm.c
> +++ b/xen/arch/x86/mm.c
> @@ -271,6 +271,18 @@ static l4_pgentry_t __read_mostly split_l4e;
>  #define root_pgt_pv_xen_slots ROOT_PAGETABLE_PV_XEN_SLOTS
>  #endif
>  
> +static void __init assign_io_page(struct page_info *page)
> +{
> +    set_gpfn_from_mfn(mfn_x(page_to_mfn(page)), INVALID_M2P_ENTRY);
> +
> +    /* The incremented type count pins as writable. */
> +    page->u.inuse.type_info = PGT_writable_page | PGT_validated | 1;
> +
> +    page_set_owner(page, dom_io);
> +
> +    page->count_info |= PGC_allocated | 1;
> +}

... clarifying its relationship with share_xen_page_with_guest().

Jan