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

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2013-03-14 16:00, Gleb Natapov wrote:
> 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.

Grmbl, likely not.

Jan

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