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

"Maciej S. Szmigiero" <mail@xxxxxxxxxxxxxxxxxxxxx> · Mon, 23 May 2022 18:26:29 +0200

On 21.05.2022 10:54, Paolo Bonzini wrote:
On Fri, May 20, 2022 at 8:33 PM Maciej S. Szmigiero
<mail@xxxxxxxxxxxxxxxxxxxxx> wrote:

From: "Maciej S. Szmigiero" <maciej.szmigiero@xxxxxxxxxx>

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 },
     },

The "feature_dependencies" way seems to work fine too,
and it definitely looks neater.

Thanks for the pointer, will post v2 in a moment.

Paolo

Thanks,
Maciej