Re: [RFC PATCH 5/9] x86/bugs: Use Virtual MSRs to request hardware mitigations

Pawan Gupta
2022-12-12T20:23:42Z

On Sun, Dec 11, 2022 at 12:00:42AM +0800, Zhang Chen wrote:
> From: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
>
> 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@linux.intel.com>
> ---
> arch/x86/include/asm/msr-index.h | 23 +++++++++++++++++++++++
> arch/x86/kernel/cpu/bugs.c | 24 ++++++++++++++++++++++++
> 2 files changed, 47 insertions(+)
>
> diff --git a/arch/x86/include/asm/msr-index.h b/arch/x86/include/asm/msr-index.h
> index 1143ac9400c3..1166b472377c 100644
> --- a/arch/x86/include/asm/msr-index.h
> +++ b/arch/x86/include/asm/msr-index.h
> @@ -165,6 +165,7 @@
> * IA32_XAPIC_DISABLE_STATUS MSR
> * supported
> */
> +#define ARCH_CAP_VIRTUAL_ENUM BIT(63) /* MSR_VIRTUAL_ENUMERATION supported */
>
> #define MSR_IA32_FLUSH_CMD 0x0000010b
> #define L1D_FLUSH BIT(0) /*
> @@ -1062,6 +1063,28 @@
> #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_RETPOLINE_S_USED BIT(1) /*
> + * Request VMM to deploy
> + * RRSBA_DIS_S mitigation
> + */
> +
> /* 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 3e3230cccaa7..a9e869f568ee 100644
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -1379,6 +1379,28 @@ static void __init spectre_v2_determine_rsb_fill_type_at_vmexit(enum spectre_v2_
> dump_stack();
> }
>
> +/* Speculation control using virtualized MSRs */
> +static void __init spec_ctrl_setup_virtualized_msr(void)
> +{
> + u64 msr_virt_enum, msr_mitigation_enum, msr_mitigation_ctrl;
> +
> + 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);
> + /* When retpoline is being used, request relevant hardware controls */
> + if (boot_cpu_has(X86_FEATURE_RETPOLINE) &&
> + msr_mitigation_enum & MITI_ENUM_RETPOLINE_S_SUPPORT) {
> + rdmsrl(MSR_VIRTUAL_MITIGATION_CTRL, msr_mitigation_ctrl);
> + msr_mitigation_ctrl |= MITI_CTRL_RETPOLINE_S_USED;
> + wrmsrl(MSR_VIRTUAL_MITIGATION_CTRL, msr_mitigation_ctrl);
> + }
> +}
> +
> static void __init spectre_v2_select_mitigation(void)
> {
> enum spectre_v2_mitigation_cmd cmd = spectre_v2_parse_cmdline();
> @@ -1485,6 +1507,8 @@ static void __init spectre_v2_select_mitigation(void)
> mode == SPECTRE_V2_RETPOLINE)
> spec_ctrl_disable_kernel_rrsba();
>
> + spec_ctrl_setup_virtualized_msr();

I think this also needs to be called during secondary CPU
initialization.