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Re: [Xen-devel] [PATCH 2/3] x86/msr: Definitions for MSR_INTEL_CORE_THREAD_COUNT


  • To: Jan Beulich <jbeulich@xxxxxxxx>
  • From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
  • Date: Mon, 15 Apr 2019 09:29:56 +0100
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  • Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx, wei.liu2@xxxxxxxxxx, roger.pau@xxxxxxxxxx
  • Delivery-date: Mon, 15 Apr 2019 08:30:53 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
  • Openpgp: preference=signencrypt

On 15/04/2019 09:09, Jan Beulich wrote:
>>>> Andrew Cooper <andrew.cooper3@xxxxxxxxxx> 04/13/19 6:22 PM >>>--- 
>>>> a/xen/arch/x86/msr.c+++ b/xen/arch/x86/msr.c
>> @@ -131,6 +131,7 @@ int guest_rdmsr(const struct vcpu *v, uint32_t msr, 
>> uint64_t *val)
>> case MSR_PRED_CMD:
>> case MSR_FLUSH_CMD:
>> /* Write-only */
>> +    case MSR_INTEL_CORE_THREAD_COUNT:
>> case MSR_TSX_FORCE_ABORT:
>> /* Not offered to guests. */
>> goto gp_fault;
> In a private talk we had, didn't you mention there is at least one OS (was it 
> OSX)
> that unconditionally ready this MSR? Despite assuming there are other issues
> with OSX, wouldn't it be better to avoid giving #GP(0) back here? I realize 
> we can't
> yet give back a fully consistent value here, looking at what conclusions Linux
> draws from other CPUID output, couldn't 0x00010001 be used as "fake" value?

I did consider that option, but for anyone looking at this MSR, it is
likely to make the situation worse rather than better.

The other option to consider is retaining the current leaky behaviour. 
There haven't been obvious problems thus far, and the accurate topology
information is going to be arriving shortly to mesh in with the core
scheduling work.

~Andrew

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