Messages in this thread | | | Subject | Re: [PATCH] x86/boot: Fix boot failure when SMP MP-table is based at 0 | From | Tom Lendacky <> | Date | Mon, 6 Nov 2017 17:00:31 -0600 |
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On 11/6/2017 4:01 PM, 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.
Btw, it was working before because instead of saving off the physical address as 5997efb96756 now does, it saved off a virtual address that pointed to physical address 0 (0xffff880000000000) and used that in the conditional.
Thanks, Tom
> > Thanks, > Tom > >>
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