Yep, that seems to fix it for me! Thanks! Gabe
On Tue, Jan 18, 2022 at 1:09 PM Volker RĂ¼melin <vr_q...@t-online.de> wrote: > > Hi! I'm a major contributor to the gem5 open source computer > > architecture simulator, and I'm trying to get SeaBIOS and FreeDOS to > > run on it. We've had at least some level of x86 support on our > > simulator for a number of years now, but we've primarily focused on 64 > > bit mode. I've found a lot of bugs in our simulator as I've been going > > along, but despite my best efforts I haven't been able to find a way > > to blame my code for the bug I'm currently stuck on. > > > > I'm using a very stripped down configuration of SeaBIOS, and am using > > the serial console to interact with it since I haven't written a > > simulated VGA interface yet. It reads FreeDOS from their published > > QEMU disk image, and it starts up and prints a menu where it wants me > > to select from 4 different boot modes. Roughly when I send a character > > over the serial connection, the simulator crashes because the software > > running on it tried to access an address that has nothing behind it. > > > > > > I've been tracing this problem down, and I see an int 0x16 with code > > 0x1 happening, which is trying to check the keyboard status, I > > believe. That goes along, and eventually calls check_irqs, which calls > > need_hop_back which returns 1, and then calls "stack_hop_back" to call > > back into itself but on the "original callers stack" I 75% know what > > that's talking about, but I'm not 100%. > > > > Anyway, once we're on the other stack, we call into the > > clock_poll_irq, that calls clock_update, that calls the (inlined I > > think) sercon_check_event, and when that tries to > > SET_LOW(rx_buf[rx_bytes], byte) the bad access happens. At least I'm > > pretty confident that's where it happens, it could also be in one of > > the other lines right around there. > > This is probably a bug in src/sercon.c. rx_bytes is marked VARLOW. The > following code is not tested but I think > > SET_LOW(rx_buf[GET_LOW(rx_bytes)], byte); > > is correct. > > I used objdump -drS -m i8086 rom16.o for the disassembly listing. > f3d8: 8e c3 mov %bx,%es > if (GET_LOW(rx_bytes) < sizeof(rx_buf)) { > f3da: 26 8a 16 79 f1 mov %es:-0xe87,%dl > f3df: 80 fa 0f cmp $0xf,%dl > f3e2: 77 e6 ja f3ca <ExtraStack+0x1f2> > SET_LOW(rx_buf[GET_LOW(rx_bytes)], byte); > f3e4: 66 0f b6 ca movzbl %dl,%ecx > f3e8: 26 67 88 81 7c f1 00 mov %al,%es:0xf17c(%ecx) > f3ef: 00 > SET_LOW(rx_bytes, GET_LOW(rx_bytes) + 1); > f3f0: 66 42 inc %edx > f3f2: 26 88 16 79 f1 mov %dl,%es:-0xe87 > count++; > f3f7: 66 46 inc %esi > f3f9: eb cf jmp f3ca <ExtraStack+0x1f2> > > With best regards, > Volker > > > > > > > The problem seems to be that the variable it's trying to access is > > supposed to be in the "e" segment, ie with selector 0xe000 and base > > address 0xe0000. The code that does this is here: > > > > fbd43: 8e c3 mov %bx,%es > > fbd45: 26 8a 16 9d f7 mov %es:-0x863,%dl > > fbd4a: 80 fa 0f cmp $0xf,%dl > > fbd4d: 77 e6 ja fbd35 <clock_update+0x6b> > > fbd4f: 66 0f b6 0e 9d f7 movzbl -0x863,%ecx <==== where it > > asplodes > > fbd55: 26 67 88 81 a0 f7 00 mov %al,%es:0xf7a0(%ecx) > > > > Note the comparison against 0xf, which I think is where it checks > > against the size of rx_buf. > > > > You can see here that this access is (I think) using the %ds register > > by default. It has an operand size prefix, and a 2 byte displacement > > of 0xf79d. Adding this to 0xe0000 gives 0xef79d, which from what I've > > seen is a pretty valid looking address, not far below where I have the > > BIOS ROM mapped in. > > > > Unfortunately when this has problems, %ds is actually 0x9d80, which > > gives a base of 0x9d800, which gives a linear address of 0xacf9d. This > > is in the middle of the (not yet implemented) VGA framebuffer which is > > why it dies. > > > > > > I then traced down why %ds has this value, and it's from the "hop > > back" step, specifically here: > > > > asm volatile( > > // Backup stack_pos and current %ss/%esp > > "movl %6, %4\n" > > "movw %%ss, %w3\n" > > "movl %%esp, %6\n" > > // Restore original callers' %ss/%esp > > "movl -4(%4), %5\n" > > "movl %5, %%ss\n" <======== Where %ss is set > > "movw %%ds:-8(%4), %%sp\n" > > "movl %5, %%ds\n" <======== Where %ds is set > > // Call func > > > > Note that in this code, *both* %ss and %ds are being set, and being > > set to the same thing. This value *was* successfully pulled off the > > saved data from when the int was originally called as far as I can > > tell, but this value of %ds does *not* seem to be correct, since the > > first time it's used it causes the bad access. > > > > > > Could you please help me figure out what's going wrong here? Is this > > supposed to work out somehow, and my simulator is just wrong (my bet, > > but what's it doing wrong?), or is this a bug in SeaBIOS? Am I using > > SeaBIOS in some way it's known not to work? > > > > Please let me know if you need any other info, I'll be more than happy > > to get this sorted out! > > > > Gabe > > > > _______________________________________________ > > SeaBIOS mailing list -- seabios@seabios.org > > To unsubscribe send an email to seabios-le...@seabios.org > >
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