Re: [PATCH v13 45/48] KVM: selftests: Introduce rdmsr_from_l2() and use it for MSR-Bitmap tests

[Vitaly Kuznetsov]

Sean Christopherson <seanjc@xxxxxxxxxx> writes:

> On Tue, Nov 01, 2022, Vitaly Kuznetsov wrote:
>> Hyper-V MSR-Bitmap tests do RDMSR from L2 to exit to L1. While 'evmcs_test'
>> correctly clobbers all GPRs (which are not preserved), 'hyperv_svm_test'
>> does not. Introduce and use common rdmsr_from_l2() to avoid code
>> duplication and remove hardcoding of MSRs.
>> 
>> Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
>> ---
>>  .../selftests/kvm/include/x86_64/processor.h  |  9 +++++++
>>  .../testing/selftests/kvm/x86_64/evmcs_test.c | 24 ++++---------------
>>  .../selftests/kvm/x86_64/hyperv_svm_test.c    |  8 +++----
>>  3 files changed, 17 insertions(+), 24 deletions(-)
>> 
>> diff --git a/tools/testing/selftests/kvm/include/x86_64/processor.h b/tools/testing/selftests/kvm/include/x86_64/processor.h
>> index fbaf0b6cec4b..a14b7e4ea7c4 100644
>> --- a/tools/testing/selftests/kvm/include/x86_64/processor.h
>> +++ b/tools/testing/selftests/kvm/include/x86_64/processor.h
>> @@ -520,6 +520,15 @@ static inline void cpu_relax(void)
>>  		"hlt\n"	\
>>  		)
>>  
>> +/* Exit to L1 from L2 with RDMSR instruction */
>> +static inline void rdmsr_from_l2(uint32_t msr)
>
> I would prefer keeping this helper out of common x86-64 code, even if it means
> duplicating code across multiple Hyper-V tests until the L1 VM-Enter/VM-Exit
> sequences get cleaned up.  The name is misleading, e.g. it doesn't really read
> the MSR since there are no outputs

It's somewhat similar to vmcall()/vmmcall() which are only used to exit
from L2 to L1 (and thus nobody complained that all the register values
are random) and not issue a hypercall and return some value.

> and while we could obviously fix that with a
> rename or a generic DO_VMEXIT_FROM_L2() macro, I would rather fix the underlying
> problem of the world switches clobbering L2 state.  That way all the helpers that
> exist for L1 can be used verbatim for L2 instead of needing dedicated helpers for
> every instruction that is used to trigger a VM-Exit.
>

I'm fine with keeping two copies of this in Hyper-V tests for now, of course.

-- 
Vitaly