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

["RVP via gnats" <gnats-admin%NetBSD.org@localhost>]

The following reply was made to PR port-amd64/59361; it has been noted by GNATS.

From: RVP <rvp%SDF.ORG@localhost>
To: gnats-bugs%netbsd.org@localhost
Cc: 
Subject: Re: port-amd64/59361: paging up in less(1) under qemu vga console
 is slow
Date: Sun, 26 Oct 2025 22:35:24 +0000 (UTC)

 On Sat, 26 Apr 2025, campbell+netbsd%mumble.net@localhost wrote:
 
 >> Description:
 > Paging _down_ in less(1) under qemu vga console is reasonably quick.  Not instantaneous -- I still see a flicker sometimes -- but quick enough.
 >
 > Paging _up_ in less(1) under qemu vga console is excruciatingly slow, takes multiple seconds to go a single page in a 25x80 display.
 >
 
 There are a few workarounds:
 
 Userspace:
 ----------
 1. `export LESS=-c' or run `less -c' to that less(1) repaints rather than scrolls
 up from the bottom. (This also works around most, not all, delays in the
 `vt100' terminfo entry.)
 
 2. Don't use `-accel nvmm' :(. (TCG, the default accel doesn't seem to suffer
     from this.)
 
 Bootloader:
 -----------
 Force use of VESA:
 boot> vesa on
 
 Kernel:
 -------
 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:
 
 ```
 --- sys/dev/ic/vga.c.orig	2021-08-07 16:19:12.000000000 +0000
 +++ sys/dev/ic/vga.c	2025-05-20 04:02:12.133432513 +0000
 @@ -544,8 +544,8 @@
   	    &vh->vh_ioh_6845))
   		panic("vga_init: couldn't map 6845 io");
 
 -	if (bus_space_map(vh->vh_memt, 0xa0000, 0x20000, 
 -	    BUS_SPACE_MAP_CACHEABLE, &vh->vh_allmemh))
 +	if (bus_space_map(vh->vh_memt, 0xa0000, 0x20000,
 +	    BUS_SPACE_MAP_PREFETCHABLE | BUS_SPACE_MAP_LINEAR, &vh->vh_allmemh))
   		panic("vga_init: couldn't map memory");
 
   	if (bus_space_subregion(vh->vh_memt, vh->vh_allmemh,
 @@ -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
   		}
   	} else
   		memcpy(&scr->pcs.mem[dstoff], &scr->pcs.mem[srcoff],
 ```
 
 HTH,
 
 -RVP