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

["Kani, Toshimitsu" <toshi.kani@xxxxxxx>]

On Tue, 2016-05-24 at 11:54 -0400, Boris Ostrovsky wrote:
> On 05/24/2016 10:53 AM, Kani, Toshimitsu wrote:
> > 
> > On Mon, 2016-05-23 at 15:52 -0700, Ed Swierk wrote:
> > > 
> > > Good question. I ran my tests again, and found I'd misinterpreted the
> > > Fusion behavior.
> > > 
> > > On Fusion 8.1.1, MSR_IA32_CR_PAT returns a reasonable value:
> > > 
> > > (XEN) Freed 308kB init memory.
> > > mapping kernel into physical memory
> > > cpu_has_pat=0 cpuid_edx(1)=f89cbf5 pat=65536
> > > pat_init_cache_modes pat=50100070406
> > > pat_init_cache_modes i=7 pat_val=0 cache=3
> > > pat_init_cache_modes ok
> > > pat_init_cache_modes i=6 pat_val=0 cache=3
> > > pat_init_cache_modes ok
> > > pat_init_cache_modes i=5 pat_val=5 cache=5
> > > pat_init_cache_modes ok
> > > pat_init_cache_modes i=4 pat_val=1 cache=1
> > > pat_init_cache_modes ok
> > > pat_init_cache_modes i=3 pat_val=0 cache=3
> > > pat_init_cache_modes ok
> > > pat_init_cache_modes i=2 pat_val=7 cache=2
> > > pat_init_cache_modes ok
> > > pat_init_cache_modes i=1 pat_val=4 cache=4
> > > pat_init_cache_modes ok
> > > pat_init_cache_modes i=0 pat_val=6 cache=0
> > > pat_init_cache_modes ok
> > > pat_init_cache_modes pat_msg=WB  WT  UC- UC  WC  WP  UC  UC
> > > about to get started...
> > > [    0.000000] x86/PAT: Configuration [0-7]: WB  WT  UC-
> > > UC  WC  WP  UC  UC
> > > 
> > > On ESXi 5.5.0, MSR_IA32_CR_PAT returns 0, and we are indeed hitting
> > > the BUG_ON in update_cache_mode_entry():
> > > 
> > > (XEN) Freed 312kB init memory.
> > > mapping kernel into physical memory
> > > cpu_has_pat=0 cpuid_edx(1)=f89cbf5 pat=65536
> > > pat_init_cache_modes pat=0
> > > pat_init_cache_modes i=7 pat_val=0 cache=3
> > > pat_init_cache_modes ok
> > > pat_init_cache_modes i=6 pat_val=0 cache=3
> > > pat_init_cache_modes ok
> > > pat_init_cache_modes i=5 pat_val=0 cache=3
> > > pat_init_cache_modes ok
> > > pat_init_cache_modes i=4 pat_val=0 cache=3
> > > pat_init_cache_modes ok
> > > pat_init_cache_modes i=3 pat_val=0 cache=3
> > > pat_init_cache_modes ok
> > > pat_init_cache_modes i=2 pat_val=0 cache=3
> > > pat_init_cache_modes ok
> > > pat_init_cache_modes i=1 pat_val=0 cache=3
> > > pat_init_cache_modes ok
> > > pat_init_cache_modes i=0 pat_val=0 cache=3
> > > (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
> > > ffff82d0802276c3
> > > create_bounce_frame+0x12b/0x13a
> > > 
> > > In both cases, the PAT CPUID feature bit is set, and cpu_has_pat is
> > > always 0 at this early point (so my RFC patch is wrong). The simplest
> > > fix is to call pat_init_cache_modes(pat) only if pat != 0.
> > > 
> > > This is starting to look like the same logic that's in
> > > pat_bsp_init(),
> > > which doesn't seem to be called when booting on Xen. Should it be?
> > > Was
> > > Xen deliberately excluded from this PAT emulation change?
> > > https://groups.google.com/d/msg/linux.kernel/JoJKbCOxV0U/PM0I9d1v60kJ
> >
> > Calling pat_init() requires the CPU rendezvous handler in MTRR, which
> > is disabled in Xen.  This PAT initialization has been problematic, and
> > the following patches addressed it in 4.6.  This will fix your problem
> > as well. 
> > https://lkml.org/lkml/2016/3/23/500
> > 
> > In particular, patch 6/7 removed the Xen code in question.
> > https://lkml.org/lkml/2016/3/23/503
> > 
> > Do you need to fix this issue in 4.4?  If so, we should be able to
> > request backporting the patches to 4.4 stable.
> 
> Would disabling PAT when the MSR is clearly broken (and not trying to
> emulate it) not work?

That should work, but the above patches fix the qemu32 issue also found in
4.4. So, they need to be backported to 4.4.
https://lkml.org/lkml/2016/3/3/828

-Toshi
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