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

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Message-ID: <93a3edef-dde6-4ef6-ae40-39990040a497@mailbox.org>
Date: Tue, 10 Dec 2024 21:33:58 +0100
From: Simon Pilkington <simonp.git@...lbox.org>
To: Sean Christopherson <seanjc@...gle.com>
Cc: linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
 regressions@...ts.linux.dev, Tom Lendacky <thomas.lendacky@....com>
Subject: Re: [REGRESSION] from 74a0e79df68a8042fb84fd7207e57b70722cf825: VFIO
 PCI passthrough no longer works

On 10/12/2024 16:47, Sean Christopherson wrote:
> Can you run with the below to see what bits the guest is trying to set (or clear)?
> We could get the same info via tracepoints, but this will likely be faster/easier.
> 
> ---
>  arch/x86/kvm/svm/svm.c | 12 +++++++++---
>  1 file changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index dd15cc635655..5144d0283c9d 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -3195,11 +3195,14 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr)
>  	case MSR_AMD64_DE_CFG: {
>  		u64 supported_de_cfg;
>  
> -		if (svm_get_feature_msr(ecx, &supported_de_cfg))
> +		if (WARN_ON_ONCE(svm_get_feature_msr(ecx, &supported_de_cfg)))
>  			return 1;
>  
> -		if (data & ~supported_de_cfg)
> +		if (data & ~supported_de_cfg) {
> +			pr_warn("DE_CFG supported = %llx, WRMSR = %llx\n",
> +				supported_de_cfg, data);
>  			return 1;
> +		}
>  
>  		/*
>  		 * Don't let the guest change the host-programmed value.  The
> @@ -3207,8 +3210,11 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr)
>  		 * 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)
> +		if (!msr->host_initiated && data != svm->msr_decfg) {
> +			pr_warn("DE_CFG current = %llx, WRMSR = %llx\n",
> +				svm->msr_decfg, data);
>  			return 1;
> +		}
>  
>  		svm->msr_decfg = data;
>  		break;
> 
> base-commit: fac04efc5c793dccbd07e2d59af9f90b7fc0dca4

Relevant dmesg output with some context below. VM locked up as expected.

[   85.834971] vfio-pci 0000:0c:00.0: resetting
[   85.937573] vfio-pci 0000:0c:00.0: reset done
[   86.494210] vfio-pci 0000:0c:00.0: resetting
[   86.494264] vfio-pci 0000:0c:00.1: resetting
[   86.761442] vfio-pci 0000:0c:00.0: reset done
[   86.761480] vfio-pci 0000:0c:00.1: reset done
[   86.762392] vfio-pci 0000:0c:00.0: resetting
[   86.865462] vfio-pci 0000:0c:00.0: reset done
[   86.977360] virbr0: port 1(vnet1) entered learning state
[   88.993052] virbr0: port 1(vnet1) entered forwarding state
[   88.993057] virbr0: topology change detected, propagating
[  103.459114] kvm_amd: DE_CFG current = 0, WRMSR = 2
[  161.442032] virbr0: port 1(vnet1) entered disabled state // VM shut down