Re: [PATCH] KVM: VMX: Fix repeatly calling alloc_apic_access_page()

[Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org>]

Sheng Yang wrote:
> From 00a52112d813af983dd4d34cb7dc701f6fe88829 Mon Sep 17 00:00:00 2001
> From: Sheng Yang <sheng.yang-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>
> Date: Fri, 2 Nov 2007 11:56:17 +0800
> Subject: [PATCH] KVM: VMX: Fix repeatly calling alloc_apic_access_page()
>
> For SMP guest, alloc_apic_access_page() would be called more than once. So 
> only the last vcpu's vmcs get correct apic access address, causing SMP guest 
> can't benifit from FlexPriority.
>
> This patch fixed this issue.
>
> Signed-off-by: Sheng Yang <sheng.yang-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>
> ---
>  drivers/kvm/kvm_main.c |    1 +
>  drivers/kvm/vmx.c      |    5 ++++-
>  2 files changed, 5 insertions(+), 1 deletions(-)
>
> diff --git a/drivers/kvm/kvm_main.c b/drivers/kvm/kvm_main.c
> index 34a681d..519626d 100644
> --- a/drivers/kvm/kvm_main.c
> +++ b/drivers/kvm/kvm_main.c
> @@ -245,6 +245,7 @@ static struct kvm *kvm_create_vm(void)
>  	spin_lock(&kvm_lock);
>  	list_add(&kvm->vm_list, &vm_list);
>  	spin_unlock(&kvm_lock);
> +	kvm->apic_access_page = NULL;
>   

Seems unnecessary, since the whole thing is kzalloc()ed?

>  	return kvm;
>  }
>  
> diff --git a/drivers/kvm/vmx.c b/drivers/kvm/vmx.c
> index 42e7fad..89007b2 100644
> --- a/drivers/kvm/vmx.c
> +++ b/drivers/kvm/vmx.c
> @@ -1466,6 +1466,8 @@ static int alloc_apic_access_page(struct kvm *kvm)
>  	int r;
>  
>  	r = -EFAULT;
> +	if (kvm->apic_access_page)
> +		return 0;
>   

Racy, what if two vcpus are created simultaneously?

>  	kvm_userspace_mem.slot = APIC_ACCESS_PAGE_PRIVATE_MEMSLOT;
>  	kvm_userspace_mem.flags = 0;
>  	kvm_userspace_mem.guest_phys_addr = 0xfee00000ULL;
> @@ -1584,7 +1586,8 @@ static int vmx_vcpu_setup(struct vcpu_vmx *vmx)
>  	vmcs_writel(CR0_GUEST_HOST_MASK, ~0UL);
>  	vmcs_writel(CR4_GUEST_HOST_MASK, KVM_GUEST_CR4_MASK);
>  
> -	if (vm_need_virtualize_apic_accesses(vmx->vcpu.kvm))
> +	if ((vmx->vcpu.vcpu_id == 0) &&
> +	    (vm_need_virtualize_apic_accesses(vmx->vcpu.kvm)))
>  		if (alloc_apic_access_page(vmx->vcpu.kvm) != 0)
>  			return -ENOMEM;
>  
>   

We may not have vcpu id 0 (though it's very unlikely).

I think the problems arise because we are doing a VM-wide operation 
(memory slot) from a vcpu context.  I think adding a ->vm_create() arch 
op and allocating the memory there will be better (under kvm->lock).

-- 
Any sufficiently difficult bug is indistinguishable from a feature.


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