Re: [Xen-devel] PAT-related crash booting Linux 4.4 + Xen 4.5 on VMware ESXi

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 05/23/2016 10:15 AM, Konrad Rzeszutek Wilk wrote:
> On Fri, May 20, 2016 at 04:58:09PM -0700, Ed Swierk wrote:
>> (XEN) traps.c:459:d0v0 Unhandled invalid opcode fault/trap [#6] on VCPU 0 
>> [ec=0000]
>> (XEN) domain_crash_sync called from entry.S: fault at ffff82d0802286c3 
>> create_bounce_frame+0x12b/0x13a
>> (XEN) Domain 0 (vcpu#0) crashed on cpu#0:
>> (XEN) ----[ Xen-4.5.4-pre  x86_64  debug=n  Not tainted ]----
>> (XEN) CPU:    0
>> (XEN) RIP:    e033:[<ffffffff81053cbd>]
>> (XEN) RFLAGS: 0000000000000206   EM: 1   CONTEXT: pv guest (d0v0)
>> (XEN) rax: 0000000000000022   rbx: 00000000ffffffff   rcx: 0000000000000000
>> (XEN) rdx: 0000000000000022   rsi: 0000000000000003   rdi: 0000000000000000
>> (XEN) rbp: ffffffff81b67ea8   rsp: ffffffff81b67e68   r8:  0000000000000001
>> (XEN) r9:  0000000000000001   r10: ffffffff81b67f20   r11: 6c61765f74617020
>> (XEN) r12: 0000000000000000   r13: 0000000000000003   r14: 0000000000000000
>> (XEN) r15: ffffffff81b67ebb   cr0: 000000008005003b   cr4: 00000000001526b0
>> (XEN) cr3: 00000001b16eb000   cr2: 0000000000000000
>> (XEN) ds: 0000   es: 0000   fs: 0000   gs: 0000   ss: e02b   cs: e033
>> (XEN) Guest stack trace from rsp=ffffffff81b67e68:
>> (XEN)    0000000000000000 6c61765f74617020 ffffffff81053cbd 000000010000e030
>> (XEN)    0000000000010006 ffffffff81b67ea8 000000000000e02b ffffffff81b67f20
>> (XEN)    ffffffff81b67f10 ffffffff8105b339 55ffffff81b67f10 5520204355202043
>> (XEN)    5520204355202043 5520204355202043 0020204355202043 0000000000000000
>> (XEN)    0000000000000000 ffffffff81b67f38 0000000000000000 0000000000000000
>> (XEN)    0000000000000000 ffffffff81b67ff0 ffffffff82010d0a 0000000000000000
>> (XEN)    000306f200000000 fed8320300010800 0000000000000000 0000000000000000
>> (XEN)    0000000000000000 0000000000000000 0000000000000000 0000000000000000
>> (XEN)    0000000000000000 0000000000000000 0000000000000000 0000000000000000
>> (XEN)    0000000000000000 0000000000000000 0000000000000000 0000000000000000
>> (XEN)    0000000000000000 0000000000000000 0000000000000000 0000000000000000
>> (XEN)    0000000000000000 ffffffff81b68008 0000000000000000 0000000000000000
>> (XEN)    0000000000000000 0000000000000000 00000000fffedb08
>> (XEN) Domain 0 crashed: rebooting machine in 5 seconds.
>> (XEN) Resetting with ACPI MEMORY or I/O RESET_REG.
>>
>> The crash occurs in pat_init_cache_modes(), called by
>> xen_start_kernel().  The pat value from MSR_IA32_CR_PAT is 0.
>> Strangely, the same kernel and Xen boot just fine on VMware Fusion
>> 8.1.1, even though the MSR is 0 there as well.

Are you hitting BUG_ON in update_cache_mode_entry()? I don't think I can
see how you can avoid it when MSR read returns 0.


>>
>> Anyway, guessing that it's pointless to call pat_init_cache_modes()
>> when the CPU doesn't support PAT, I added a check for cpu_has_pat.
>> This resolves the problem on ESXi and doesn't seem to break real
>> hardware, though I'm not sure how to verify PAT functionality.  So
>> this is just an RFC.

Can you start an HVM guest in Xen after your patch below?

> Cc-ing maintainers.
>> diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
>> index 9a29803..209f680 100644
>> --- a/arch/x86/xen/enlighten.c
>> +++ b/arch/x86/xen/enlighten.c
>> @@ -1633,8 +1633,12 @@ asmlinkage __visible void __init 
>> xen_start_kernel(void)
>>       * Modify the cache mode translation tables to match Xen's PAT
>>       * configuration.
>>       */
>> -    rdmsrl(MSR_IA32_CR_PAT, pat);
>> -    pat_init_cache_modes(pat);
>> +    if (cpu_has_pat) {
>> +            rdmsrl(MSR_IA32_CR_PAT, pat);
>> +            pat_init_cache_modes(pat);
>> +    } else {
>> +            xen_raw_console_write("CPU does not support PAT\n");
>> +    }
>>  
>>      /* keep using Xen gdt for now; no urgent need to change it */
>>

This looks OK to me but I think we should first understand why you don't
crash on Fusion.

Also, PAT initialization code has been rewritten in Linux (for 4.5?) so
I suspect this problem is only observed on earlier kernels.

-boris


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