Anthony Liguori <anth...@codemonkey.ws> writes: > Markus Armbruster <arm...@redhat.com> writes: > >> Avi Kivity <a...@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.
Okay, I'll cook up a patch fixing pc_cmos_init().