Re: [RFC 22/23] xen/privcmd: Add support for Linux 64KB page granularity

[David Vrabel <david.vrabel@citrix.com>]

On 14/05/15 18:01, Julien Grall wrote:
> The hypercall interface (as well as the toolstack) is always using 4KB
> page granularity. When the toolstack is asking for mapping a series of
> guest PFN in a batch, it expects to have the page map contiguously in
> its virtual memory.
> 
> When Linux is using 64KB page granularity, the privcmd driver will have
> to map multiple Xen PFN in a single Linux page.
> 
> Note that this solution works on page granularity which is a multiple of
> 4KB.
[...]
> --- a/drivers/xen/xlate_mmu.c
> +++ b/drivers/xen/xlate_mmu.c
> @@ -63,6 +63,7 @@ static int map_foreign_page(unsigned long lpfn, unsigned long fgmfn,
>  
>  struct remap_data {
>  	xen_pfn_t *fgmfn; /* foreign domain's gmfn */
> +	xen_pfn_t *egmfn; /* end foreign domain's gmfn */

I don't know what you mean by "end foreign domain".

>  	pgprot_t prot;
>  	domid_t  domid;
>  	struct vm_area_struct *vma;
> @@ -78,17 +79,23 @@ static int remap_pte_fn(pte_t *ptep, pgtable_t token, unsigned long addr,
>  {
>  	struct remap_data *info = data;
>  	struct page *page = info->pages[info->index++];
> -	unsigned long pfn = page_to_pfn(page);
> -	pte_t pte = pte_mkspecial(pfn_pte(pfn, info->prot));
> +	unsigned long pfn = xen_page_to_pfn(page);
> +	pte_t pte = pte_mkspecial(pfn_pte(page_to_pfn(page), info->prot));
>  	int rc;
> -
> -	rc = map_foreign_page(pfn, *info->fgmfn, info->domid);
> -	*info->err_ptr++ = rc;
> -	if (!rc) {
> -		set_pte_at(info->vma->vm_mm, addr, ptep, pte);
> -		info->mapped++;
> +	uint32_t i;
> +
> +	for (i = 0; i < XEN_PFN_PER_PAGE; i++) {
> +		if (info->fgmfn == info->egmfn)
> +			break;
> +
> +		rc = map_foreign_page(pfn++, *info->fgmfn, info->domid);
> +		*info->err_ptr++ = rc;
> +		if (!rc) {
> +			set_pte_at(info->vma->vm_mm, addr, ptep, pte);
> +			info->mapped++;
> +		}
> +		info->fgmfn++;

This doesn't make any sense to me.  Don't you need to gather the foreign
GFNs into batches of PAGE_SIZE / XEN_PAGE_SIZE and map these all at once
into a 64 KiB page?  I don't see how you can have a set_pte_at() for
each foreign GFN.

David