Re: [PATCH] KVM: unbreak userspace that does not sets tss address

[Marcelo Tosatti <mtosatti@redhat.com>]

On Mon, Mar 07, 2011 at 05:19:36PM +0200, Gleb Natapov wrote:
> Commit 6440e5967bc broke old userspaces that do not set tss address
> before entering vcpu. Unbreak it by setting tss address to a safe
> value on the first vcpu entry. New userspaces should set tss address,
> so print warning in case it doesn't.
> 
> Signed-off-by: Gleb Natapov <gleb@redhat.com>
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index e2b8c6b..280b6ea 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -180,6 +180,7 @@ static u64 construct_eptp(unsigned long root_hpa);
>  static void kvm_cpu_vmxon(u64 addr);
>  static void kvm_cpu_vmxoff(void);
>  static void vmx_set_cr3(struct kvm_vcpu *vcpu, unsigned long cr3);
> +static int vmx_set_tss_addr(struct kvm *kvm, unsigned int addr);
>  
>  static DEFINE_PER_CPU(struct vmcs *, vmxarea);
>  static DEFINE_PER_CPU(struct vmcs *, current_vmcs);
> @@ -1762,6 +1763,16 @@ static void enter_rmode(struct kvm_vcpu *vcpu)
>  	vmx->emulation_required = 1;
>  	vmx->rmode.vm86_active = 1;
>  
> +	/*
> +	 * Very old userspace does not call KVM_SET_TSS_ADDR before entering
> +	 * vcpu. Call it here with phys address pointing 16M below 4G.
> +	 */
> +	if (!vcpu->kvm->arch.tss_addr) {
> +		printk_once(KERN_WARNING "kvm: KVM_SET_TSS_ADDR need to be "
> +			     "called before entering vcpu\n");
> +		vmx_set_tss_addr(vcpu->kvm, 0xfeffd000);
> +	}
> +
>  	vmx->rmode.tr.selector = vmcs_read16(GUEST_TR_SELECTOR);
>  	vmx->rmode.tr.base = vmcs_readl(GUEST_TR_BASE);
>  	vmcs_writel(GUEST_TR_BASE, rmode_tss_base(vcpu->kvm));

Should leave/enter srcu section before/after set_memory_region. 

Actually, for older userspace, isnt the base_gfn + slot[0].npages - 3
address (rmode_tss_base) used? That is, initializing the TSS page at 
that address at vcpu_reset time should be enough?