Re: [PATCH 0/2] KVM: x86: Propagate AMD-specific IBRS bits to guests

[Paolo Bonzini]

On 2/27/23 22:40, Borislav Petkov wrote:
> On Mon, Feb 27, 2023 at 09:05:24PM +0000, Takahiro Itazuri wrote:
> > VMMs retrieve supported CPUID features via KVM_GET_SUPPORTED_CPUID to
> > construct CPUID information to be passed to KVM_SET_CPUID2. Most CPUID
> > feature bits related to speculative attacks are propagated from host
> > CPUID. But AMD processors have AMD-specific IBRS related bits in CPUID
> > Fn8000_0008_EBX (ref: AMD64 Architecture Programmer's Manual Volume 3:
> > General-Purpose and System Instructions) and some bits are not
> > propagated to guests.
> >
> > Enable propagation of these bits to guests, so that VMMs don't have to
> > enable them explicitly based on host CPUID.
>
> How hard is it for the VMMs to enable them?

Let me rephrase the second paragraph of Takahiro's commit message:

"Tell the VMMs that they can pass the bits to the guests, instead of 
having to second-guess that the hypervisor does not have to do anything 
to support these bits".

In general, userspace should not second guess the hypervisor.  There are 
some rare cases in which QEMU (and probably the proprietary hypervisors 
at Google and Amazon) does that, but in general you want it to trust 
information coming from the kernel.  New CPUID bits are quite frequent, 
and sometimes also stupidly difficult to get right, so if filtering 
CPUID can be done in the kernel you won't have to do the same change N 
times in _all_ userspaces that use KVM.

Paolo