drivers: xen: gnttab: process gentabs in reverse order

[GrygiriiS]

The Xen extends domain gentab region every time domain requests gentab basing on gentab idx. If idx > xen_current_max_gentab_idx the Xen extends number of gentabs so that idx <= xen_current_max_gentab_idx. As result now when gentabs are requested from gentab low_idx to max_idx Xen is performing gentab extension by 1 and the bunch of log messages is produced:

(XEN) xen-source/xen/common/grant_table.c:1909:d0v0 Expanding d0
grant table from 1 to 2 frames

This patch changes gentab processing from gentab max_idx to low_idx which allows Xen to perform gentab extension only once (to max_idx) and only one message will be produced in the log:

(XEN) xen-source/xen/common/grant_table.c:1909:d0v0 Expanding d0
grant table from 1 to 64 frames

[GrygiriiS]

@firscity @lorc @Deedone @oleksiimoisieiev @xakep-amatop

[lorc]

(XEN) xen-source/xen/common/grant_table.c:1909:d0v0 Expanding d0 \
  grant table from 1 to 2 frames

This message is being printed only if Xen is being build in debug mode. So it is not a problem in production mode.

I don't believe that this patch will get into Zephyr mainline. You can try, though.

But as you are trying to include it into rpi5_dev branch, I have no objections.

Acked-by: Volodymyr Babchuk <[email protected]>

[GrygiriiS]

The Linux Kernel has the following:

static int gnttab_map(unsigned int start_idx, unsigned int end_idx)
{
...
	unsigned int nr_gframes = end_idx + 1;
	int rc;

	if (xen_feature(XENFEAT_auto_translated_physmap)) {
		struct xen_add_to_physmap xatp;
		unsigned int i = end_idx;
		rc = 0;
		BUG_ON(xen_auto_xlat_grant_frames.count < nr_gframes);
		/*
		 * Loop backwards, so that the first hypercall has the largest
		 * index, ensuring that the table will grow only once.
		 */
		do {
			xatp.domid = DOMID_SELF;
			xatp.idx = i;
			xatp.space = XENMAPSPACE_grant_table;
			xatp.gpfn = xen_auto_xlat_grant_frames.pfn[i];
			rc = HYPERVISOR_memory_op(XENMEM_add_to_physmap, &xatp);

So, nothing special here really - just a bit faster and less logs.

[lorc]

		/*
		 * Loop backwards, so that the first hypercall has the largest
		 * index, ensuring that the table will grow only once.
		 */

This is a better justification than "avoid printing debug logs". I believe, if you'll rewrite the commit message WRT to this, it would be much better.

[firscity]

Also in addition to @lorc comment, please, use gnttab (grant table) instead of gentab

[firscity]

Reviewed-by: Dmytro Firsov <[email protected]>