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Re: [Xen-devel] [PATCH] x86/apic: Improve current_local_apic_mode()


  • To: Jan Beulich <jbeulich@xxxxxxxx>
  • From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
  • Date: Fri, 14 Feb 2020 17:59:49 +0000
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  • Cc: Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
  • Delivery-date: Fri, 14 Feb 2020 18:00:02 +0000
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  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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On 28/01/2020 14:10, Jan Beulich wrote:
> On 28.01.2020 13:52, Andrew Cooper wrote:
>> boot_cpu_has(X86_FEATURE_X2APIC) doesn't need checking to interpret
>> APIC_BASE_EXTD.
> Hmm, the comment you remove ...
>
>> --- a/xen/arch/x86/apic.c
>> +++ b/xen/arch/x86/apic.c
>> @@ -1534,18 +1534,14 @@ void __init record_boot_APIC_mode(void)
>>  /* Look at the bits in MSR_APIC_BASE and work out which APIC mode we are in 
>> */
>>  enum apic_mode current_local_apic_mode(void)
>>  {
>> -    u64 msr_contents;
>> +    uint32_t high, low;
>>  
>> -    rdmsrl(MSR_APIC_BASE, msr_contents);
>> +    rdmsr(MSR_APIC_BASE, low, high);
>>  
>> -    /* Reading EXTD bit from the MSR is only valid if CPUID
>> -     * says so, else reserved */
> ... states the situation correctly, I think. I guess there's no hardware
> allowing the bit to be set without the feature being there, but a virtual
> or emulated environment could go and set the bit without violating any
> specification, as long as the CPUID bit is clear.

It is unrealistic to expect that some emulated environment supports
preserving of a reserved bit when real hardware uses #GP.

> (Afaict we still allow
> PV guests to see the host MSR_APIC_BASE contents, yet such guests
> wouldn't see the CPUID flag set.

I tried an experiment a few years ago to properly reject access to
MSR_APIC_BASE for PV guests.  Suffice it to say that Linux doesn't boot.

This is ultimately a bug in Linux, stemming from broken MSR handling in
Xen, and inappropriate leakage of state which shouldn't ever have been
available to PV guests.

PV guests cannot interact with the LAPIC at all.  Their only interrupt
controller is the event channel interface.

> We've had a customer inferring from the
> set bit in the MSR that the other x2APIC [host] MSRs can also be read.

Right, but that is bugs stacked on top of bugs.  Its not surprising
there is a cascade set of failures.

Also remember that PV guests get to see the host's x2apic setting,
seemingly for topology reasons.  I'm not convinced this is actually a
clever idea, but given that noone has actually fixed guest topology
handling yet, I've also gone out of my way not to make changes in this
area while adjusting other aspects of CPUID handling.

~Andrew

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