Re: [PATCH v3] target/i386/kvm: Refine VMX controls setting for backward compatibility

[Ewan Hai <ewanhai-oc@zhaoxin.com>]

Sorry for my oversight, I am adding the maintainers who were
missed in the previous email.

On 6/24/24 05:58, EwanHai wrote:
> Commit 4a910e1 ("target/i386: do not set unsupported VMX secondary
> execution controls") implemented a workaround for hosts that have
> specific CPUID features but do not support the corresponding VMX
> controls, e.g., hosts support RDSEED but do not support RDSEED-Exiting.
>
> In detail, commit 4a910e1 introduced a flag `has_msr_vmx_procbased_clts2`.
> If KVM has `MSR_IA32_VMX_PROCBASED_CTLS2` in its msr list, QEMU would
> use KVM's settings, avoiding any modifications to this MSR.
>
> However, this commit (4a910e1) didn't account for cases in older Linux
> kernels(4.17~5.2) where `MSR_IA32_VMX_PROCBASED_CTLS2` is in
> `kvm_feature_msrs`-obtained by ioctl(KVM_GET_MSR_FEATURE_INDEX_LIST),
> but not in `kvm_msr_list`-obtained by ioctl(KVM_GET_MSR_INDEX_LIST).
> As a result,it did not set the `has_msr_vmx_procbased_clts2` flag based
> on `kvm_msr_list` alone, even though KVM does maintain the value of
> this MSR.
>
> This patch supplements the above logic, ensuring that
> `has_msr_vmx_procbased_clts2` is correctly set by checking both MSR
> lists, thus maintaining compatibility with older kernels.
>
> Signed-off-by: EwanHai <ewanhai-oc@zhaoxin.com>
> ---
> Changes in v3:
> - Use a more precise version range in the comment, specifically "4.17~5.2"
> instead of "<5.3".
>
> Changes in v2:
> - Adjusted some punctuation in the commit message as per suggestions.
> - Added comments to the newly added code to indicate that it is a compatibility fix.
>
> v1 link:
> https://lore.kernel.org/all/20230925071453.14908-1-ewanhai-oc@zhaoxin.com/
>
> v2 link:
> https://lore.kernel.org/all/20231127034326.257596-1-ewanhai-oc@zhaoxin.com/
> ---
>   target/i386/kvm/kvm.c | 15 +++++++++++++++
>   1 file changed, 15 insertions(+)
>
> diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c
> index 7ad8072748..a7c6c5b2d0 100644
> --- a/target/i386/kvm/kvm.c
> +++ b/target/i386/kvm/kvm.c
> @@ -2386,6 +2386,7 @@ void kvm_arch_do_init_vcpu(X86CPU *cpu)
>   static int kvm_get_supported_feature_msrs(KVMState *s)
>   {
>       int ret = 0;
> +    int i;
>   
>       if (kvm_feature_msrs != NULL) {
>           return 0;
> @@ -2420,6 +2421,20 @@ static int kvm_get_supported_feature_msrs(KVMState *s)
>           return ret;
>       }
>   
> +   /*
> +    * Compatibility fix:
> +    * Older Linux kernels (4.17~5.2) report MSR_IA32_VMX_PROCBASED_CTLS2
> +    * in KVM_GET_MSR_FEATURE_INDEX_LIST but not in KVM_GET_MSR_INDEX_LIST.
> +    * This leads to an issue in older kernel versions where QEMU,
> +    * through the KVM_GET_MSR_INDEX_LIST check, assumes the kernel
> +    * doesn't maintain MSR_IA32_VMX_PROCBASED_CTLS2, resulting in
> +    * incorrect settings by QEMU for this MSR.
> +    */
> +    for (i = 0; i < kvm_feature_msrs->nmsrs; i++) {
> +        if (kvm_feature_msrs->indices[i] == MSR_IA32_VMX_PROCBASED_CTLS2) {
> +            has_msr_vmx_procbased_ctls2 = true;
> +        }
> +    }
>       return 0;
>   }
>