9fans archive / 1994 / 03 / 4 /    prev next

From: Sean Quinlan sean@CS....
Subject: No subject
Date: Tue, 1 Mar 1994 19:47:03 -0500


There is a problem with the plan9 memory management system for the PC
due to one of the brain dead features of the 386 architecture.
In particular, the R/W bit in a user page table entry is ignored when
the processor is in supervisor mode.  This means that copy on write will
not work when the kernel is copying into user space.
For example the following code segment exhibits the problem.

#include <u.h>
#include <libc.h>

char    buf[1024];

main()
{
        int i;

        if (fork() == 0) {
                /* make sure memory is allocated RONLY */
                i = buf[0];

                read(0, buf, 1024);
                print("%s\n", buf);
                exits(0);
        }

        /* wait a second so the child completes */

        sleep(1000);

        buf[1023] = 0;

        print("parent -> %s\n", buf);
}

On plan 9 for the pc, the data read into buf will incorrectly appear in both
the child and parent process.

The 486 (and higher?) fixed the problem but to maintain compatibility
it defaults to the 386 behavior.  On a 486 you need to set bit 16 in CR0
to get the correct behavior for the R/W bit.  Of course, if you have a
386, then copy on write can not be fixed, but you can still run plan9
with a copy on reference scheme... unfortunately, this means your 386
machine runs slower and requires more memory...

The complete fix of the problem, thanks to Dave Presotto, follows:

Here's the changes.  It works on a couple of 486's and 386's so its
probably right.

In l.s (around line 131) change

        ORL     $0X80000000,AX
        ANDL    $~(0x8|0x2),AX  /* TS=0, MP=0 */

to

        ORL     $0X80010000,AX
        ANDL    $~(0x40000000|0x20000000|0x8|0x2),AX    /* CD=0, NW=0, TS=0, MP=0 */

This also turns on internal caching in case it was off.

In l.s add the routine

/*
 *  return cpu type (what is a pentium?)
 */
TEXT    x86(SB),$0

        PUSHFL
        MOVL    0(SP),AX
        ORL     $0x40000,AX
        PUSHL   AX
        POPFL
        PUSHFL
        POPL    AX
        ANDL    $0x40000,AX
        JZ      is386
        MOVL    $486,AX
        JMP     done
is386:
        MOVL    $386,AX
done:
        POPFL
        RET

This returns the number 386 or 486 depending.  I guess it'll return
586 also when I get a chance at a Pentium.

In fns.h add a definition for it:

        int     x86(void);

In main.c in confinit() change

        conf.copymode = 0;      /* copy on write */

to

        switch(x86()){
        case 386:
                conf.copymode = 1;      /* copy on reference */
                break;
        default:
        case 486:
                conf.copymode = 0;      /* copy on write */
                break;
        }

and you're done.