Re: [PATCH] target/i386/kvm: Fix disabling MPX on "-cpu host" with MPX-capable host

[Paolo Bonzini]

On Fri, May 20, 2022 at 8:33 PM Maciej S. Szmigiero
<mail@maciej.szmigiero.name> wrote:
>
> From: "Maciej S. Szmigiero" <maciej.szmigiero@oracle.com>
>
> Since KVM commit 5f76f6f5ff96 ("KVM: nVMX: Do not expose MPX VMX controls 
> when guest MPX disabled")
> it is not possible to disable MPX on a "-cpu host" just by adding "-mpx"
> there if the host CPU does indeed support MPX.
> QEMU will fail to set MSR_IA32_VMX_TRUE_{EXIT,ENTRY}_CTLS MSRs in this case
> and so trigger an assertion failure.
>
> Instead, besides "-mpx" one has to explicitly add also
> "-vmx-exit-clear-bndcfgs" and "-vmx-entry-load-bndcfgs" to QEMU command
> line to make it work, which is a bit convoluted.
>
> Sanitize MPX-related bits in MSR_IA32_VMX_TRUE_{EXIT,ENTRY}_CTLS after
> setting the vCPU CPUID instead so such workarounds are no longer necessary.

Can you use feature_dependencies instead? See for example

    {
        .from = { FEAT_7_0_EBX,             CPUID_7_0_EBX_RDSEED },
        .to = { FEAT_VMX_SECONDARY_CTLS,    VMX_SECONDARY_EXEC_RDSEED_EXITING },
    },

Paolo