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

[Markus Armbruster]

Anthony Liguori <address@hidden> writes:

> Markus Armbruster <address@hidden> writes:
>
>> Avi Kivity <address@hidden> 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.

Okay, I'll cook up a patch fixing pc_cmos_init().