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

[Binbin Wu]

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?


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


> +#define MITI_CTRL_RETPOLINE_S_USED_BIT		1	/*
> +							 * Request VMM to deploy
> +							 * RRSBA_DIS_S mitigation
> +							 */
> +#define MITI_CTRL_RETPOLINE_S_USED		BIT(MITI_CTRL_RETPOLINE_S_USED_BIT)
> +
>   /* AMD-V MSRs */
>   
>   #define MSR_VM_CR                       0xc0010114
> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> index f9d060e71c3e..5326c03d9d5e 100644
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -1435,6 +1435,27 @@ static void __init spectre_v2_determine_rsb_fill_type_at_vmexit(enum spectre_v2_
>   	dump_stack();
>   }
>   
> +/* Speculation control using virtualized MSRs */
> +static void spec_ctrl_setup_virtualized_msr(void)
> +{
> +	u64 msr_virt_enum, msr_mitigation_enum;
> +
> +	/* When retpoline is being used, request relevant hardware controls */
> +	if (!boot_cpu_has(X86_FEATURE_RETPOLINE))
> +		return;
> +
> +	if (!(x86_read_arch_cap_msr() & ARCH_CAP_VIRTUAL_ENUM))
> +		return;
> +
> +	rdmsrl(MSR_VIRTUAL_ENUMERATION, msr_virt_enum);
> +	if (!(msr_virt_enum & VIRT_ENUM_MITIGATION_CTRL_SUPPORT))
> +		return;
> +
> +	rdmsrl(MSR_VIRTUAL_MITIGATION_ENUM, msr_mitigation_enum);
> +	if (msr_mitigation_enum & MITI_ENUM_RETPOLINE_S_SUPPORT)
> +		msr_set_bit(MSR_VIRTUAL_MITIGATION_CTRL, MITI_CTRL_RETPOLINE_S_USED_BIT);
> +}
> +
>   static void __init spectre_v2_select_mitigation(void)
>   {
>   	enum spectre_v2_mitigation_cmd cmd = spectre_v2_parse_cmdline();
> @@ -1546,6 +1567,8 @@ static void __init spectre_v2_select_mitigation(void)
>   	    mode == SPECTRE_V2_RETPOLINE)
>   		spec_ctrl_disable_kernel_rrsba();
>   
> +	spec_ctrl_setup_virtualized_msr();
> +
>   	spectre_v2_enabled = mode;
>   	pr_info("%s\n", spectre_v2_strings[mode]);
>   
> @@ -2115,6 +2138,8 @@ void x86_spec_ctrl_setup_ap(void)
>   
>   	if (ssb_mode == SPEC_STORE_BYPASS_DISABLE)
>   		x86_amd_ssb_disable();
> +
> +	spec_ctrl_setup_virtualized_msr();
>   }
>   
>   bool itlb_multihit_kvm_mitigation;