Re: [PATCH v2] KVM: x86: Mask off unsupported and unknown bits of IA32_ARCH_CAPABILITIES

[Xiaoyao Li <xiaoyao.li@intel.com>]

On 8/31/2022 1:49 AM, Jim Mattson wrote:
> KVM should not claim to virtualize unknown IA32_ARCH_CAPABILITIES
> bits. When kvm_get_arch_capabilities() was originally written, there
> were only a few bits defined in this MSR, and KVM could virtualize all
> of them. However, over the years, several bits have been defined that
> KVM cannot just blindly pass through to the guest without additional
> work (such as virtualizing an MSR promised by the
> IA32_ARCH_CAPABILITES feature bit).
> 
> Define a mask of supported IA32_ARCH_CAPABILITIES bits, and mask off
> any other bits that are set in the hardware MSR.

Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>

> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Fixes: 5b76a3cff011 ("KVM: VMX: Tell the nested hypervisor to skip L1D flush on vmentry")
> Signed-off-by: Jim Mattson <jmattson@google.com>
> Reviewed-by: Vipin Sharma <vipinsh@google.com>
> ---
> 
>   v1 -> v2: Clarified comment about unsupported bits.
> 
>   arch/x86/kvm/x86.c | 25 +++++++++++++++++++++----
>   1 file changed, 21 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 205ebdc2b11b..9a18acfcfdc8 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1557,12 +1557,32 @@ static const u32 msr_based_features_all[] = {
>   static u32 msr_based_features[ARRAY_SIZE(msr_based_features_all)];
>   static unsigned int num_msr_based_features;
>   
> +/*
> + * Some IA32_ARCH_CAPABILITIES bits have dependencies on MSRs that KVM
> + * does not yet virtualize. These include:
> + *   10 - MISC_PACKAGE_CTRLS
> + *   11 - ENERGY_FILTERING_CTL
> + *   12 - DOITM
> + *   18 - FB_CLEAR_CTRL
> + *   21 - XAPIC_DISABLE_STATUS
> + *   23 - OVERCLOCKING_STATUS
> + */
> +
> +#define KVM_SUPPORTED_ARCH_CAP \
> +	(ARCH_CAP_RDCL_NO | ARCH_CAP_IBRS_ALL | ARCH_CAP_RSBA | \
> +	 ARCH_CAP_SKIP_VMENTRY_L1DFLUSH | ARCH_CAP_SSB_NO | ARCH_CAP_MDS_NO | \
> +	 ARCH_CAP_PSCHANGE_MC_NO | ARCH_CAP_TSX_CTRL_MSR | ARCH_CAP_TAA_NO | \
> +	 ARCH_CAP_SBDR_SSDP_NO | ARCH_CAP_FBSDP_NO | ARCH_CAP_PSDP_NO | \
> +	 ARCH_CAP_FB_CLEAR | ARCH_CAP_RRSBA | ARCH_CAP_PBRSB_NO)
> +
>   static u64 kvm_get_arch_capabilities(void)
>   {
>   	u64 data = 0;
>   
> -	if (boot_cpu_has(X86_FEATURE_ARCH_CAPABILITIES))
> +	if (boot_cpu_has(X86_FEATURE_ARCH_CAPABILITIES)) {
>   		rdmsrl(MSR_IA32_ARCH_CAPABILITIES, data);
> +		data &= KVM_SUPPORTED_ARCH_CAP;
> +	}
>   
>   	/*
>   	 * If nx_huge_pages is enabled, KVM's shadow paging will ensure that
> @@ -1610,9 +1630,6 @@ static u64 kvm_get_arch_capabilities(void)
>   		 */
>   	}
>   
> -	/* Guests don't need to know "Fill buffer clear control" exists */
> -	data &= ~ARCH_CAP_FB_CLEAR_CTRL;
> -
>   	return data;
>   }
>