On 12/10/24 09:47, Sean Christopherson wrote:
> +Tom
>
> On Tue, Dec 10, 2024, Simon Pilkington wrote:
>> Hi,
>>
>> With the aforementioned commit I am no longer able to use PCI passthrough to
>> a Windows guest on the X570 chipset with a 5950X CPU.
>>
>> The minimal reproducer for me is to attach a GPU to the VM and attempt to
>> start Windows setup from an iso image. The VM will apparently livelock at the
>> setup splash screen before the spinner appears as one of my CPU cores goes up
>> to 100% usage until I force off the VM. This could be very machine-specific
>> though.
>
> Ugh. Yeah, it's pretty much guaranteed to be CPU specific behavior.
>
> Tom, any idea what the guest might be trying to do? It probably doesn't matter
> in the end, it's not like KVM does anything with the value...
No clue. I do see that in Linux there is a zenbleed-related bit that is
used in DE_CFG:
522b1d69219d ("x86/cpu/amd: Add a Zenbleed fix")
I wonder if it might be related to that.
Your suggestion below to see what bits are being requested might shed
some light on it.
Thanks,
Tom
>
>> Reverting to the old XOR check fixes both 6.12.y stable and 6.13-rc2 for me.
>> Otherwise they're both bad. Can you please look into it? I can share the
>> config I used for test builds if it would help.
>
> 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
