Re: [REGRESSION] from 74a0e79df68a8042fb84fd7207e57b70722cf825: VFIO PCI passthrough no longer works

[Simon Pilkington <simonp.git@mailbox.org>]

On 10/12/2024 23:43, Sean Christopherson wrote:
> Unless you (Tom) disagree, I vote to simply drop the offending code, i.e. make
> all supported bits fully writable from the guest.  KVM is firmly in the wrong here,
> and I can't think of any reason to disallow the guest from clearing LFENCE_SERIALIZE.
> 
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 6a350cee2f6c..5a82ead3bf0f 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -3201,15 +3201,6 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr)
>                 if (data & ~supported_de_cfg)
>                         return 1;
>  
> -               /*
> -                * Don't let the guest change the host-programmed value.  The
> -                * MSR is very model specific, i.e. contains multiple bits that
> -                * are completely unknown to KVM, and the one bit known to KVM
> -                * is simply a reflection of hardware capabilities.
> -                */
> -               if (!msr->host_initiated && data != svm->msr_decfg)
> -                       return 1;
> -
>                 svm->msr_decfg = data;
>                 break;
>         }
> 

This also produces a good kernel.