Messages in this thread | | | Date | Fri, 17 Nov 2017 14:03:40 +0100 (CET) | From | Thomas Gleixner <> | Subject | Re: [PATCH] x86/boot: Fix boot failure when SMP MP-table is based at 0 |
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On Mon, 6 Nov 2017, Tom Lendacky wrote: > On 11/6/2017 3:41 PM, H. Peter Anvin wrote: > > On 11/06/17 12:17, Tom Lendacky wrote: > > > When crosvm is used to boot a kernel as a VM, the SMP MP-table is found > > > at physical address 0x0. This causes mpf_base to be set to 0 and a > > > subsequent "if (!mpf_base)" check in default_get_smp_config() results in > > > the MP-table not being parsed. Further into the boot this results in an > > > oops when attempting a read_apic_id(). > > > > > > Add a boolean variable that is set to true when the MP-table is found. > > > Use this variable for testing if the MP-table was found so that even a > > > value of 0 for mpf_base will result in continued parsing of the MP-table. > > > > > > Reported-by: Tomeu Vizoso <tomeu@tomeuvizoso.net> > > > Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com> > > > > Ahem... did anyone ever tell you that this is an epicly bad idea on your > > part? The low megabyte of physical memory has very special meaning on > > x86, and deviating from the standard use of this memory is a *very* > > dangerous thing to do, and imposing on the kernel a "fake null pointer" > > requirement that exists only for the convenience of your particular > > brokenness is not okay. > > > > -hpa > > That was my initial thought... what was something doing down at the start > of memory. But when I looked at default_find_smp_config() it specifically > scans the bottom 1K for a an MP-table signature. I was hoping to get some > feedback as to whether this would really be an acceptable thing to do. So > I'm good with this patch being rejected, but the change I made in > > 5997efb96756 ("x86/boot: Use memremap() to map the MPF and MPC data") > > does break something that was working before.
This goes back to Linux 1.1.x
/* * Physical page 0 is special; it's not touched by Linux since BIOS * and SMM (for laptops with [34]86/SL chips) may need it. It is read * and write protected to detect null pointer references in the * kernel. * It may also hold the MP configuration table when we are booting SMP. */
and then got changed in 2.3.40 to:
/* * FIXME: Linux assumes you have 640K of base ram.. * this continues the error... * * 1) Scan the bottom 1K for a signature * 2) Scan the top 1K of base RAM * 3) Scan the 64K of bios */ if (smp_scan_config(0x0, 0x400) || smp_scan_config(639 * 0x400, 0x400) || smp_scan_config(0xF0000, 0x10000)) return;
which is what we still have completely unmodified.
What hpa considers to be epic fail was added 20+ years ago and probably with a reason. We carried that along forever and now someone looked for a place to stick MP config in for his VM thingy and picked the first place which is checked. Not a big surprise.
So we are stuck with it, whether we like it or not.
Thanks,
tglx
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