Re: [PATCH] kvm: x86: Add Intel PMU MSRs to msrs_to_save[]

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Sean Christopherson <sean.j.christopherson@intel.com> writes:

> On Fri, Sep 27, 2019 at 05:19:25PM +0200, Paolo Bonzini wrote:
>> On 27/09/19 16:40, Vitaly Kuznetsov wrote:
>> > Paolo Bonzini <pbonzini@redhat.com> writes:
>> > 
>> >> On 27/09/19 15:53, Vitaly Kuznetsov wrote:
>> >>> Paolo Bonzini <pbonzini@redhat.com> writes:
>> >>>
>> >>>> Queued, thanks.
>> >>>
>> >>> I'm sorry for late feedback but this commit seems to be causing
>> >>> selftests failures for me, e.g.:
>> >>>
>> >>> # ./x86_64/state_test 
>> >>> Testing guest mode: PA-bits:ANY, VA-bits:48,  4K pages
>> >>> Guest physical address width detected: 46
>> >>> ==== Test Assertion Failure ====
>> >>>   lib/x86_64/processor.c:1089: r == nmsrs
>> >>>   pid=14431 tid=14431 - Argument list too long
>> >>>      1	0x000000000040a55f: vcpu_save_state at processor.c:1088 (discriminator 3)
>> >>>      2	0x00000000004010e3: main at state_test.c:171 (discriminator 4)
>> >>>      3	0x00007f881eb453d4: ?? ??:0
>> >>>      4	0x0000000000401287: _start at ??:?
>> >>>   Unexpected result from KVM_GET_MSRS, r: 36 (failed at 194)
>
> That "failed at %x" print line should really be updated to make it clear
> that it's printing hex...
>

Yea, I also wasn't sure and had to look in the code. Will send a patch
if no one beats me to it.

>> >>> Is this something known already or should I investigate?
>> >>
>> >> No, I didn't know about it, it works here.
>> >>
>> > 
>> > Ok, this is a bit weird :-) '194' is 'MSR_ARCH_PERFMON_EVENTSEL0 +
>> > 14'. In intel_pmu_refresh() nr_arch_gp_counters is set to '8', however,
>> > rdmsr_safe() for this MSR passes in kvm_init_msr_list() (but it fails
>> > for 0x18e..0x193!) so it stay in the list. get_gp_pmc(), however, checks
>> > it against nr_arch_gp_counters and returns a failure.
>> 
>> Huh, 194h apparently is a "FLEX_RATIO" MSR.  I agree that PMU MSRs need
>> to be checked against CPUID before allowing them.
>
> My vote would be to programmatically generate the MSRs using CPUID and the
> base MSR, as opposed to dumping them into the list and cross-referencing
> them against CPUID.  E.g. there should also be some form of check that the
> architectural PMUs are even supported.

Yes. The problem appears to be that msrs_to_save[] and emulated_msrs[]
are global and for the MSRs in question we check
kvm_find_cpuid_entry(vcpu, 0xa, ) to find out how many of them are
available so this can be different for different VMs (and even vCPUs :-)
However,

"KVM_GET_MSR_INDEX_LIST returns the guest msrs that are supported.  The list
varies by kvm version and host processor, but does not change otherwise."

So it seems that PMU MSRs just can't be there. Revert?

-- 
Vitaly