Re: [Qemu-devel] Funny -m arguments can crash

[Anthony Liguori <anthony@codemonkey.ws>]

Markus Armbruster <armbru@redhat.com> writes:

> Avi Kivity <avi@redhat.com> writes:
>
>> On 08/08/2012 12:04 PM, Markus Armbruster wrote:
>>>>
>>>> Yes please, maybe with a notice to the user.
>>> 
>>> Next problem: minimum RAM size.
>>> 
>>> For instance, -M pc -m X, where X < 32KiB dies "qemu: fatal: Trying to
>>> execute code outside RAM or ROM at [...] Aborted (core dumped)" with
>>> TCG, and "KVM internal error. Suberror: 1" with KVM.
>>> 
>>> Should a minimum RAM size be enforced?  Board-specific?
>>> 
>>
>> It's really a BIOS bug causing a limitation of both kvm and tcg to be
>> hit.  The BIOS should recognize it doesn't have sufficient memory and
>> hang gracefully (if you can picture that).  It just assumes some low
>> memory is available and tries to execute it with the results you got.
>
> SeaBIOS indeed assumes it got at least 1MiB of RAM.  It doesn't bother
> to check CMOS for a smaller RAM size.  However, that bug / feature is
> currently masked by a QEMU bug: we screw up CMOS contents when there's
> less than 1 MiB of RAM.  pc_cmos_init():
>
>     int val, nb, i;
> [...]
>     /* memory size */
>     val = 640; /* base memory in K */
>     rtc_set_memory(s, 0x15, val);
>     rtc_set_memory(s, 0x16, val >> 8);
>
>     val = (ram_size / 1024) - 1024;
>     if (val > 65535)
>         val = 65535;
>     rtc_set_memory(s, 0x17, val);
>     rtc_set_memory(s, 0x18, val >> 8);
>
> If ram_size < 1MiB, val goes negative.  Oops.
>
> For instance, with -m 500k, we happily promise 640KiB base memory (CMOS
> addr 0x15..16), almost 64MiB extended memory (0x17..18 and 0x30..31),
> yet no memory above 16MiB (0x34..35).
>
> An easy way to fix this is to require 1MiB of RAM :)
>
> But if you like, I'll put sane values in CMOS instead.  That'll expose
> the SeaBIOS bug.
>
> Anthony, you're the PC maintainer, got a preference?
>
> SeaBIOS thread:
> http://comments.gmane.org/gmane.comp.bios.coreboot.seabios/4341

I'd prefer fixing the CMOS values over limiting to 1MB of RAM.

Having a 1MB limit is purely theoritical--not practical.  There's no
good reason for anyone to ask for < 1MB unless they know what they're
doing.  If it's truly a mistake, then asking for 2MB is just as much of
a mistake because no real guest will run with 2MB of memory anyway (you
can't even load a kernel).

So if we're just going for theoritical correctness, we ought to do it
the Right Way which is fixing the CMOS values and putting the check in
SeaBIOS.

Regards,

Anthony Liguori