HATAYAMA Daisuke <d.hatayama at jp.fujitsu.com> writes: > From: "H. Peter Anvin" <hpa at zytor.com> > Subject: Re: [PATCH v1 2/2] x86, apic: Disable BSP if boot cpu is AP > Date: Mon, 22 Oct 2012 17:35:47 -0700 > >> On 10/22/2012 02:29 PM, Eric W. Biederman wrote: >>>> >>>> As I said, I thought Fenghua tried that but it didn't work, >>>> experimentally. >>> >>> Fair enough. You described the problem with clearing bit 8 in a weird >>> way. >>> >>> If the best we can muster are fuzzy memories it may be worth >>> revisiting. >>> Perhaps it works on enough cpu models to be interesting. >>> >> >> It isn't fuzzy memories... this was done as late as 1-2 months ago. I >> just don't know the details. >> >> Fenghua, could you help fill us in? >> > > I overlooked completely the fact that BSP flag is rewritable. > > I tried Eric's suggestion using attached test programs and saw it > worked fine at least on the three cpus around me below: > > - Intel(R) Xeon(R) CPU E7- 4820 @ 2.00GHz > - Intel(R) Xeon(R) CPU E7- 8870 @ 2.40GHz > - Intel(R) Xeon(TM) CPU 1.80GHz > - 32 bits CPU > > Next I found the description about this in 8.4.2, IASDM Vol.3: > > The MP initialization protocol imposes the following requirements > and restrictions on the system: > > * The MP protocol is executed only after a power-up or RESET. If the > MP protocol has completed and a BSP is chosen, subsequent INITs > (either to a specific processor or system wide) do not cause the > MP protocol to be repeated. Instead, each logical processor > examines its BSP flag (in the IA32_APIC_BASE MSR) to determine > whether it should execute the BIOS boot-strap code (if it is the > BSP) or enter a wait-for-SIPI state (if it is an AP). > > So this is no longer undocumented behaviour for recent cpus, I think. The underdocumented bit is the ability to clear the flag. And of course these are processor specific registers. > Considering these, I'll make a patch to clear BSP flag at appropreate > position in kernel boot-up code. OTOH, according to the discussion, it > was reported that clearing BSP flag affected some BIOSes. To deal with > this, I'll prepare a kernel option to decide whether to clear BSP flag > or not. > > Does anyone have any comments now? Or please comment after I submit a > new patch. I think you are on right track with preparing some patches, and this certainly looks like worth experimenting with. At least for i386 the code need to verify you have a cpu new enough to have an APIC_BASE_MSR, but I don't think that is going to be hard. Eric
