Re: [RFC PATCH v2 05/11] x86/bugs: Use Virtual MSRs to request hardware mitigations

[Chao Gao]

On Mon, Apr 17, 2023 at 09:43:59PM +0800, Binbin Wu wrote:
>
>On 4/14/2023 2:25 PM, Chao Gao wrote:
>> From: Pawan Gupta <pawan.kumar.gupta@xxxxxxxxxxxxxxx>
>> 
>> Guests that have different family/model than the host may not be aware
>> of hardware mitigations(such as RRSBA_DIS_S) available on host. This is
>> particularly true when guests migrate. To solve this problem Intel
>> processors have added a virtual MSR interface through which guests can
>> report their mitigation status and request VMM to deploy relevant
>> hardware mitigations.
>> 
>> Use this virtualized MSR interface to request relevant hardware controls
>> for retpoline mitigation.
>> 
>> Signed-off-by: Pawan Gupta <pawan.kumar.gupta@xxxxxxxxxxxxxxx>
>> Co-developed-by: Zhang Chen <chen.zhang@xxxxxxxxx>
>> 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/include/asm/msr-index.h | 25 +++++++++++++++++++++++++
>>   arch/x86/kernel/cpu/bugs.c       | 25 +++++++++++++++++++++++++
>>   2 files changed, 50 insertions(+)
>> 
>> diff --git a/arch/x86/include/asm/msr-index.h b/arch/x86/include/asm/msr-index.h
>> index 60b25d87b82c..aec213f0c6fc 100644
>> --- a/arch/x86/include/asm/msr-index.h
>> +++ b/arch/x86/include/asm/msr-index.h
>> @@ -166,6 +166,7 @@
>>   						 * IA32_XAPIC_DISABLE_STATUS MSR
>>   						 * supported
>>   						 */
>> +#define ARCH_CAP_VIRTUAL_ENUM		BIT_ULL(63) /* MSR_VIRTUAL_ENUMERATION supported */
>>   #define MSR_IA32_FLUSH_CMD		0x0000010b
>>   #define L1D_FLUSH			BIT(0)	/*
>> @@ -1103,6 +1104,30 @@
>>   #define MSR_IA32_VMX_MISC_INTEL_PT                 (1ULL << 14)
>>   #define MSR_IA32_VMX_MISC_VMWRITE_SHADOW_RO_FIELDS (1ULL << 29)
>>   #define MSR_IA32_VMX_MISC_PREEMPTION_TIMER_SCALE   0x1F
>> +
>> +/* Intel virtual MSRs */
>> +#define MSR_VIRTUAL_ENUMERATION			0x50000000
>> +#define VIRT_ENUM_MITIGATION_CTRL_SUPPORT	BIT(0)	/*
>> +							 * Mitigation ctrl via virtual
>> +							 * MSRs supported
>> +							 */
>> +
>> +#define MSR_VIRTUAL_MITIGATION_ENUM		0x50000001
>> +#define MITI_ENUM_BHB_CLEAR_SEQ_S_SUPPORT	BIT(0)	/* VMM supports BHI_DIS_S */
>> +#define MITI_ENUM_RETPOLINE_S_SUPPORT		BIT(1)	/* VMM supports RRSBA_DIS_S */
>> +
>> +#define MSR_VIRTUAL_MITIGATION_CTRL		0x50000002
>> +#define MITI_CTRL_BHB_CLEAR_SEQ_S_USED_BIT	0	/*
>> +							 * Request VMM to deploy
>> +							 * BHI_DIS_S mitigation
>> +							 */
>> +#define MITI_CTRL_BHB_CLEAR_SEQ_S_USED		BIT(MITI_CTRL_BHB_CLEAR_SEQ_S_USED_BIT)
>
>Seems it is defined, but not used to request VMM to deploy BHI_DIS_S
>mitigation?

Because Linux kernel doesn't use BHB-clearing sequence. Instead,
"disable unprivileged eBPF by default" + SMAP + eIBRS are used.

KVM uses this bit when checking if guests, which may not be running
Linux, are using BHB-clearing sequence.

>
>
>And IMO, it is more natual to put this patch after the four capability
>advertising patches.

Makes sense. I will organize the series in that order.