On 06/26/2010 12:20 AM, Josh Triplett wrote: > On Fri, Jun 25, 2010 at 02:28:01PM -0700, Josh Triplett wrote: > >> As I originally reported in http://bugs.debian.org/584846 , and later >> tracked down, grub2's int15 hook, used to augment functions e820/e801/88 >> for additional reserved memory (such as for drivemap), fails to return >> its error status correctly, causing recent Linux (and possibly other >> OSes) to detect only 64MB via BIOS-88, ignoring e820 and e801. >> >> This occurs because the interrupt must return a status in CF, but after >> using clc or stc to clear or set CF, it calls iret, which restores the >> flags from the stack. Since recent Linux has CF set when it calls >> int15, CF remains set when the interrupt returns, indicating an error. >> Linux's detect_memory_e820 and detect_memory_e801 functions thus cannot >> retrieve any memory map, and Linux must fall back to function 88, which >> cannot return more than 64MB. >> >> To reproduce this problem, just use the "drivemap" command and then boot >> a recent Linux kernel (git commit >> c549e71d073a6e9a4847497344db28a784061455 or newer, meaning v2.6.30-rc1 >> or newer). In my case, I booted a USB drive with grub2 on it, used >> "drivemap -s (hd1) (hd0)", then chainloaded the hard drive. >> >> The following patch against latest bzr fixes this problem. (Thanks to >> H. Peter Anvin for noticing a bug in my original patch and suggesting a >> fix.) With this patch, Linux's memory detection can use e820 again, and >> detect all memory. >> > Revised patch, incorporating suggestion from phcoder to eliminate the > unnecessary jmp right before the iret_cf label: > > Comitted. Thanks > === modified file 'mmap/i386/pc/mmap_helper.S' > --- mmap/i386/pc/mmap_helper.S 2010-03-26 23:04:14 +0000 > +++ mmap/i386/pc/mmap_helper.S 2010-06-25 22:16:53 +0000 > @@ -59,7 +59,7 @@ > movw %bx, %dx > pop %ds > clc > - iret > + jmp LOCAL (iret_cf) > > LOCAL (h88): > popf > @@ -69,7 +69,7 @@ > movw DS (LOCAL (kbin16mb)), %ax > pop %ds > clc > - iret > + jmp LOCAL (iret_cf) > > LOCAL (e820): > popf > @@ -101,12 +101,18 @@ > mov $0x534d4150, %eax > pop %ds > clc > - iret > + jmp LOCAL (iret_cf) > LOCAL (errexit): > mov $0x534d4150, %eax > pop %ds > + xor %bx, %bx > stc > - xor %bx, %bx > + > +LOCAL (iret_cf): > + push %bp > + mov %sp, %bp > + setc 6(%bp) > + pop %bp > iret > > VARIABLE(grub_machine_mmaphook_mmap_num) > > > ChangeLog entry for this patch: > > > 2010-06-25 Josh Triplett <j...@joshtriplett.org> > > * mmap/i386/pc/mmap_helper.S: Preserve CF by propagating it into the > flags on the stack before popping them with iret. Add a new iret_cf > label implementing this logic, and make the e820, e801, and 88 > handlers jump to it after setting or clearing CF. This fixes memory > detection with recent Linux (git commit > c549e71d073a6e9a4847497344db28a784061455 or newer, meaning v2.6.30-rc1 > or newer), which sets CF on entry to int15, and treats the preserved > CF on exit as an error in e820 and e801; without this change, it can > only successfully use BIOS-88 and detect only up to 64MB of RAM. > Originally reported as Debian bug #584846. > > > - Josh Triplett > > _______________________________________________ > Grub-devel mailing list > Grub-devel@gnu.org > http://lists.gnu.org/mailman/listinfo/grub-devel > >
-- Regards Vladimir 'φ-coder/phcoder' Serbinenko
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