linux-kernel - Re: [PATCH] KVM: SVM: Clear the CR4 register on reset

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Message-ID: <454cb7d4-9f50-42e1-6908-d659491ba140@redhat.com>
Date:   Tue, 2 Mar 2021 20:39:28 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Babu Moger <babu.moger@....com>
Cc:     wanpengli@...cent.com, kvm@...r.kernel.org, seanjc@...gle.com,
        joro@...tes.org, x86@...nel.org, linux-kernel@...r.kernel.org,
        mingo@...hat.com, bp@...en8.de, hpa@...or.com, vkuznets@...hat.com,
        tglx@...utronix.de, jmattson@...gle.com
Subject: Re: [PATCH] KVM: SVM: Clear the CR4 register on reset

On 02/03/21 19:51, Babu Moger wrote:
> This problem was reported on a SVM guest while executing kexec.
> Kexec fails to load the new kernel when the PCID feature is enabled.
> 
> When kexec starts loading the new kernel, it starts the process by
> resetting the vCPU's and then bringing each vCPU online one by one.
> The vCPU reset is supposed to reset all the register states before the
> vCPUs are brought online. However, the CR4 register is not reset during
> this process. If this register is already setup during the last boot,
> all the flags can remain intact. The X86_CR4_PCIDE bit can only be
> enabled in long mode. So, it must be enabled much later in SMP
> initialization.  Having the X86_CR4_PCIDE bit set during SMP boot can
> cause a boot failures.
> 
> Fix the issue by resetting the CR4 register in init_vmcb().
> 
> Signed-off-by: Babu Moger <babu.moger@....com>
> ---
>   arch/x86/kvm/svm/svm.c |    1 +
>   1 file changed, 1 insertion(+)
> 
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index c636021b066b..baee91c1e936 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -1200,6 +1200,7 @@ static void init_vmcb(struct vcpu_svm *svm)
>   	init_sys_seg(&save->ldtr, SEG_TYPE_LDT);
>   	init_sys_seg(&save->tr, SEG_TYPE_BUSY_TSS16);
>   
> +	svm_set_cr4(&svm->vcpu, 0);
>   	svm_set_efer(&svm->vcpu, 0);
>   	save->dr6 = 0xffff0ff0;
>   	kvm_set_rflags(&svm->vcpu, X86_EFLAGS_FIXED);
> 

Queued, thanks.

Paolo