On Mon, Aug 13, 2012 at 04:56:53PM +0300, Avi Kivity wrote: > On 08/13/2012 04:41 PM, Markus Armbruster wrote: > > 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. > > IMO we need to fix CMOS reporting. > > (technically we shouldn't touch CMOS NVRAM at all; seabios should > discover memory size via fwcfg and program it itself. But it's > pointless to change it now) > Chipset we emulate does not support all those crazy memory values you can give to -m.
-- Gleb.