Re: [REGRESSION] from 74a0e79df68a8042fb84fd7207e57b70722cf825: VFIO PCI passthrough no longer works
From: Simon Pilkington
Date: Wed Dec 11 2024 - 03:54:28 EST
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.