Subject: Re: linux-next regression: SNP Guest boot hangs with certain cpu/mem config combination

From: "Aithal, Srikanth" <[email-masked]>
Date: Wed, 26 Mar 2025 19:47:05 +0530

On 3/25/2025 7:03 PM, Kirill A. Shutemov wrote:
> On Tue, Mar 25, 2025 at 02:40:00PM +0530, Aithal, Srikanth wrote:
>> Hello,
>>
>>
>> Starting linux-next build next-20250312, including recent build 20250324, we
>> are seeing an issue where the SNP guest boot hangs at the "boot smp config"
>> step:
>>
>>
>>   [ 2.294722] smp: Bringing up secondary CPUs ...
>> [    2.295211] smpboot: Parallel CPU startup disabled by the platform
>> [    2.309687] smpboot: x86: Booting SMP configuration:
>> [    2.310214] .... node  #0, CPUs:          #1   #2   #3   #4 #5   #6
>> #7   #8   #9  #10  #11  #12  #13  #14  #15  #16  #17 #18  #19  #20  #21
>> #22  #23  #24  #25  #26  #27  #28  #29  #30 #31  #32  #33  #34  #35  #36
>> #37  #38  #39  #40  #41  #42  #43 #44  #45  #46  #47  #48  #49  #50  #51
>> #52  #53  #54  #55  #56 #57  #58  #59  #60  #61  #62  #63  #64  #65  #66
>> #67  #68  #69 #70  #71  #72  #73  #74  #75  #76  #77  #78  #79  #80  #81
>> #82 #83  #84  #85  #86  #87  #88  #89  #90  #91  #92  #93  #94  #95 #96
>> #97  #98  #99 #100 #101 #102 #103 #104 #105 #106 #107 #108 #109 #110 #111
>> #112 #113 #114 #115 #116 #117 #118 #119 #120 #121 #122 #123 #124 #125 #126
>> #127 #128 #129 #130 #131 #132 #133 #134 #135 #136 #137 #138 #139 #140 #141
>> #142 #143 #144 #145 #146 #147 #148 #149 #150 #151 #152 #153 #154 #155 #156
>> #157 #158 #159 #160 #161 #162 #163 #164 #165 #166 #167 #168 #169 #170 #171
>> #172 #173 #174 #175 #176 #177 #178 #179 #180 #181 #182 #183 #184 #185 #186
>> #187 #188 #189 #190 #191 #192 #193 #194 #195 #196 #197 #198
>> --> The guest hangs forever at this point.
>>
>>
>> I have observed that certain vCPU and memory combinations work, while others
>> do not. The VM configuration I am using does not have any NUMA nodes.
>>
>> vcpus             Mem        SNP guest boot
>> <=240            19456M    Boots fine
>>> =241,<255   19456M    Hangs
>> 1-255              2048M    Boots fine
>> 1-255              4096M    Boots fine
>>> 71                 8192M    Hangs
>>> 41                 6144M    Hangs
>>
>> When I bisected this issue, it pointed to the following commit :
>>
>>
>> *commit 800f1059c99e2b39899bdc67a7593a7bea6375d8*
>> Author: Kirill A. Shutemov <[email-masked]>
>> Date:   Mon Mar 10 10:28:55 2025 +0200
>>
>>      mm/page_alloc: fix memory accept before watermarks gets initialized
> 
> Hm. It is puzzling for me. I don't see how this commit can cause the hang.
> 
> Could you track down where hang happens?

I am attaching the full guest serial log when this issue is hit. As 
mentioned earlier guest boot hangs at smp config step.