Re: [Xen-devel] Crash with nested HVM and Linux v5.1+

[Jan Beulich <jbeulich@xxxxxxxx>]

On 21.09.2019 01:14, Sarah Newman wrote:
> With nestedhvm=1, the L2 HVM guest is either hanging (Xen 4.8) or crashing 
> (Xen 4.12.1) the L1 Xen hypervisor with recent versions of Linux. We 
> isolated the commit to:
> 
> commit 093ae8f9a86a974c920b613860f1f7fd5bbd70ab
> Author: Borislav Petkov <bp@xxxxxxx>
> Date:   Thu Apr 12 13:11:36 2018 +0200

Thanks for doing the bisection on the Linux side. Would you please also
provide the actual data about the L1 Xen crash (i.e. a maximum
verbosity L1 log of a debug build of Xen)? It might also turn out
helpful if you could provide the corresponding L0 Xen log, assuming you
have host access.

Jan

>      x86/TSC: Use RDTSCP
> 
>      Currently, the kernel uses
> 
>        [LM]FENCE; RDTSC
> 
>      in the timekeeping code, to guarantee monotonicity of time where the
>      *FENCE is selected based on vendor.
> 
>      Replace that sequence with RDTSCP which is faster or on-par and gives
>      the same guarantees.
> 
>      A microbenchmark on Intel shows that the change is on-par.
> 
>      On AMD, the change is either on-par with the current LFENCE-prefixed
>      RDTSC or slightly better with RDTSCP.
> 
>      The comparison is done with the LFENCE-prefixed RDTSC (and not with the
>      MFENCE-prefixed one, as one would normally expect) because all modern
>      AMD families make LFENCE serializing and thus avoid the heavy MFENCE by
>      effectively enabling X86_FEATURE_LFENCE_RDTSC.
> 
> I could not find RDTSCP instruction being used by Linux before the given 
> commit, which is present in Linux v5.1 and newer. As expected, masking off 
> the RDTSCP cpuid flag in leaf 0x80000001 prevents a cooperative guest from 
> using that instruction and therefore prevents the crash.
> 
> --Sarah
> 
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> 


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