On Sunday, October 23, 2011 1:57:59 pm Andriy Gapon wrote: > on 23/10/2011 18:27 Dennis Koegel said the following: > > On Fri, Oct 21, 2011 at 04:33:38PM -0400, John Baldwin wrote: > >> Working offline with Dennis, we found that changing the CFLAGS in > >> sys/boot/i386/gptboot/Makefile from "-O1" to "-Os -mrtd" (partially > >> reverting > >> an earlier commit) fixed gptboot. The next test for someone to do would > >> be to > >> try just adding "-mrtd" and leaving "-O1" as-is to see if that fixes it. > > > > More test results: > > > > gcc -Os -fno-guess-branch-probability -fomit-frame-pointer > > -fno-unit-at-a-time \ > > -mno-align-long-strings -mrtd [from before r225530]: Boots OK > > gcc -Os -mrtd: Boots OK > > gcc -O1 -mrtd: Fails > > gcc -O1: Fails > > gcc -O0: Fails > > gcc -Os: Boots OK > > > > clang -O1: Fails > > clang -Os: Fails > > clang -Oz: Fails > > > > I've put some printf()s into gpt{,boot}.c to trace where the reboot is > > triggered. It appears to be in drvsize() (called from gptread()). OTOH > > the debug output may have changed where the problem occurs, I don't > > know about that. > > > > With 9.0R drawing near, CFLAGS should be s/-O1/-Os/, until we can figure > > out what happens. But as for why gcc's magic -Os is required and clang's > > output doesn't work at all, I'm clueless. > > Thank you for your very valuable analysis! > I looked at a difference in assembly code of the drvsize function produced by > gcc -Os and by gcc -O1. One thing that was immediately obvious is that gcc > places the params array and the sectors variable in a different order for > different options. One idea is that if BIOS actually writes beyond the end of > the array, then in one case it could be harmless (overwrites the sector > variable), but in the other case it could be more harmful. > I found a document that suggests a possibility of BIOS writing more bytes to > the > array than its current size of 0x42: > http://www.t13.org/documents/UploadedDocuments/docs2008/e08134r1-BIOS_Enhanced_Disk_Drive_Services_4.0.pdf > > Of course, the size of the array is passed to BIOS at the start of the array > and > so a _non-buggy_ BIOS should not write beyond the array, but we live in a > non-perfect world.
Hmm, I think we've had to do a similar workaround in the past for a different BIOS call (SMAP maybe?). However, I do have one request, can we declare an actual structure instead of a silly char array? Then we can remove the weird casts with offsets into it, etc. Having an actual struct would be far more readable and less bug-prone. -- John Baldwin _______________________________________________ freebsd-current@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-current To unsubscribe, send any mail to "freebsd-current-unsubscr...@freebsd.org"