Re: [PATCH] KVM: VMX: Do not overwrite vcpu->srcu_idx in vmx_vcpu_reset

[Gleb Natapov <gleb@redhat.com>]

On Thu, Mar 14, 2013 at 03:52:11PM +0100, Jan Kiszka wrote:
> vmx_vcpu_reset may now be called while already holding the srcu lock, so
> we may overwrite what was already saved there. Also, we lock and unlock
> in the same context, thus there was no need to save to the vcpu anyway.
> 
> Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
> ---
> 
> Marcelo just suggested this as the simplest fix for the issue caused by
> the INIT/SIPI patch. Avoiding srcu lock for TSS handling might still be
> possible but more tricky.
> 
>  arch/x86/kvm/vmx.c |    5 +++--
>  1 files changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index 958ac3a..be5b1dc 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -4117,6 +4117,7 @@ static void vmx_vcpu_reset(struct kvm_vcpu *vcpu)
>  {
>  	struct vcpu_vmx *vmx = to_vmx(vcpu);
>  	u64 msr;
> +	int idx;
>  
>  	vmx->rmode.vm86_active = 0;
>  
> @@ -4190,9 +4191,9 @@ static void vmx_vcpu_reset(struct kvm_vcpu *vcpu)
>  		vmcs_write16(VIRTUAL_PROCESSOR_ID, vmx->vpid);
>  
>  	vmx->vcpu.arch.cr0 = X86_CR0_NW | X86_CR0_CD | X86_CR0_ET;
> -	vcpu->srcu_idx = srcu_read_lock(&vcpu->kvm->srcu);
> +	idx = srcu_read_lock(&vcpu->kvm->srcu);
>  	vmx_set_cr0(&vmx->vcpu, kvm_read_cr0(vcpu)); /* enter rmode */
> -	srcu_read_unlock(&vcpu->kvm->srcu, vcpu->srcu_idx);
> +	srcu_read_unlock(&vcpu->kvm->srcu, idx);
vmx_set_cr0() does:
                srcu_read_unlock(&vcpu->kvm->srcu, vcpu->srcu_idx);
                vmx_set_tss_addr(vcpu->kvm, 0xfeffd000);
                vcpu->srcu_idx = srcu_read_lock(&vcpu->kvm->srcu);
So with this change the sequence will be:

vcpu->srcu_idx = srcu_read_lock()
  idx = srcu_read_lock(&vcpu->kvm->srcu); 
   srcu_read_unlock(&vcpu->kvm->srcu, vcpu->srcu_idx);
   vcpu->srcu_idx = srcu_read_lock(&vcpu->kvm->srcu);
  srcu_read_unlock(&vcpu->kvm->srcu, idx);
srcu_read_unlock(&vcpu->kvm->srcu, vcpu->srcu_idx);

Not sure this is valid.

>  	vmx_set_cr4(&vmx->vcpu, 0);
>  	vmx_set_efer(&vmx->vcpu, 0);
>  	vmx_fpu_activate(&vmx->vcpu);
> -- 
> 1.7.3.4

--
			Gleb.