Re: port-amd64/59361: paging up in less(1) under qemu vga console is slow

[Taylor R Campbell <riastradh%NetBSD.org@localhost>]

> Date: Sun, 26 Oct 2025 22:35:24 +0000 (UTC)
> From: RVP <rvp%SDF.ORG@localhost>
> 
> Scrolling up (erasure of 1st line, followed by moving the rest of the lines up)
> seems to do a backwards copy. Use plain memmove(3) instead:
>  
> @@ -1168,10 +1168,15 @@
>   				    scr->pcs.cursorrow, scr->pcs.cursorcol);
>   #endif
>   		} else {
> +			bus_space_handle_t dst = memh + scr->pcs.dispoffset + dstoff * 2;
> +			bus_space_handle_t src = memh + scr->pcs.dispoffset + srcoff * 2;
> +			memmove((void* )dst, (void* )src, nrows * ncols * 2);
> +#if 0
>   			bus_space_copy_region_2(memt, memh,
>   			    scr->pcs.dispoffset + srcoff * 2,
>   			    memh, scr->pcs.dispoffset + dstoff * 2,
>   			    nrows * ncols);
> +#endif

Thanks, I bet bus_space_copy_region_2 is slow under nvmm because it
has to do a VM exit for every pair of bytes copied, and VM exits are
costly:

    811 	if (x86_bus_space_is_io(t)) {
    812 		if (addr1 >= addr2) {
    813 			/* src after dest: copy forward */
    814 			for (; c != 0; c--, addr1 += 2, addr2 += 2)
    815 				outw(addr2, inw(addr1));
    816 		} else {
    817 			/* dest after src: copy backwards */
    818 			for (addr1 += 2 * (c - 1), addr2 += 2 * (c - 1);
    819 			    c != 0; c--, addr1 -= 2, addr2 -= 2)
    820 				outw(addr2, inw(addr1));
    821 		}

https://nxr.netbsd.org/xref/src/sys/arch/x86/x86/bus_space.c?r=1.47#811

I don't think there's any I/O port equivalent of REP MOV, so there
might not be a better way to do this properly (and still support VGA
in I/O space rather than memory-mapped, as well as machines where
access must be 2-bytes-at-a-time) other than a larger temporary
buffer.