Re: [RFC PATCH v2 02/11] KVM: x86: Advertise CPUID.7.2.EDX and RRSBA_CTRL support

[Chao Gao]

On Mon, May 15, 2023 at 02:53:07PM +0800, Xiaoyao Li wrote:
>On 4/14/2023 2:25 PM, Chao Gao wrote:
>> From: Zhang Chen <chen.zhang@xxxxxxxxx>
>> 
>> Add a kvm-only CPUID feature leaf for CPUID.7.2.EDX and RRSBA_CTRL
>> as the first feature in the leaf.
>> 
>> RRSBA_CTRL is enumerated by CPUID.7.2.EDX[2]. If supported, RRSBA_DIS_U
>> (bit 5) and RRSBA_DIS_S (bit 6) of IA32_SPEC_CTRL MSR can be used to
>> disable RRSBA behavior for CPL3 and CPL0/1/2 respectively.
>> 
>> Note that KVM does not intercept guests' IA32_SPEC_CTRL MSR accesses
>> after a non-zero is written to the MSR. Therefore, guests can already
>> toggle the two bits if the host supports RRSBA_CTRL, and no extra code
>> is needed to allow guests to toggle the two bits.
>
>This is a bug that also matters with other bits in MSR_IA32_SPEC_CTRL which
>has a dedicated enumeration CPUID bit and no support in KVM yet.

Do you mean passing through the MSR is a bug? guest can write any hardware
supported value to the MSR if the MSR isn't intercepted.

I guess this is intentional and a trade-off for performance (note that
context-switch may cause writes to the MSR). And see

commit 841c2be09fe4 ("kvm: x86: replace kvm_spec_ctrl_test_value with runtime test on the host")

it appears that this behavior is widely recognized.

>
>I think we need to fix this bug at first.

I have no idea how to fix the "bug" without intercepting the MSR. The
performance penalty makes me think intercepting the MSR is not a viable
solution.

>
>> Signed-off-by: Zhang Chen <chen.zhang@xxxxxxxxx>
>> Signed-off-by: Chao Gao <chao.gao@xxxxxxxxx>
>> Tested-by: Jiaan Lu <jiaan.lu@xxxxxxxxx>
>> ---
>>   arch/x86/kvm/cpuid.c         | 22 +++++++++++++++++++---
>>   arch/x86/kvm/reverse_cpuid.h |  7 +++++++
>>   2 files changed, 26 insertions(+), 3 deletions(-)
>> 
>> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
>> index 9583a110cf5f..f024c3ac2203 100644
>> --- a/arch/x86/kvm/cpuid.c
>> +++ b/arch/x86/kvm/cpuid.c
>> @@ -685,6 +685,10 @@ void kvm_set_cpu_caps(void)
>>   		SF(SGX1) | SF(SGX2) | SF(SGX_EDECCSSA)
>>   	);
>> +	kvm_cpu_cap_init_kvm_defined(CPUID_7_2_EDX,
>> +		SF(RRSBA_CTRL)
>> +	);
>> +
>
>Please move this hook up to right follow the leaf CPUID_7_1_EAX.

sure. will do.