linux-kernel - Re: [PATCH v4] KVM: nVMX: nested TPR shadow/threshold emulation

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Date:	Wed, 20 Aug 2014 09:42:47 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Wanpeng Li <wanpeng.li@...ux.intel.com>
CC:	Jan Kiszka <jan.kiszka@...mens.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Gleb Natapov <gleb@...nel.org>, Bandan Das <bsd@...hat.com>,
	Zhang Yang <yang.z.zhang@...el.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4] KVM: nVMX: nested TPR shadow/threshold emulation

Il 20/08/2014 08:59, Wanpeng Li ha scritto:
> 
> +	/*
> +	 * Failing the vm entry is _not_ what the processor does
> +	 * but it's basically the only possibility we have.

	 * We could still enter the guest if CR8 load exits are 
	 * enabled, CR8 store exits are enabled, and virtualize APIC
	 * access is disabled; in this case the processor would never
	 * use the TPR shadow and we could simply clear the bit from
	 * the execution control.  But such a configuration is useless,
	 * so let's keep the code simple.

> +	 */
> +	if (!vmx->nested.virtual_apic_page)
> +		nested_vmx_failValid(vcpu, VMXERR_ENTRY_INVALID_CONTROL_FIELD);

I thought so, but I'm afraid it's too late to do nested_vmx_failValid 
here.

Without a test case, I'd be more confident if you moved the 
nested_release_page/nested_get_page to a separate function, that 
nested_vmx_run calls before enter_guest_mode.  The same function can 
map apic_access_page too, for cleanliness.  Something like this:

         if (cpu_has_secondary_exec_ctrls() &&
             nested_cpu_has(vmcs12,
                            CPU_BASED_ACTIVATE_SECONDARY_CONTROLS) &&
             (vmcs12->secondary_vm_exec_control &
              SECONDARY_EXEC_VIRTUALIZE_APIC_ACCESSES)) {
                        if (vmx->nested.apic_access_page) /* shouldn't happen */
                                nested_release_page(vmx->nested.apic_access_page);
                        vmx->nested.apic_access_page =
                                nested_get_page(vcpu, vmcs12->apic_access_addr);
         }
        if (...) {
	         /* do the same for virtual_apic_page if CPU_BASED_TPR_SHADOW
	            is set... */

		/*
		 * Failing the vm entry is _not_ what the processor does
		 * but it's basically the only possibility we have.
		 * We could still enter the guest if CR8 load exits are 
		 * enabled, CR8 store exits are enabled, and virtualize APIC
		 * access is disabled; in this case the processor would never
		 * use the TPR shadow and we could simply clear the bit from
		 * the execution control.  But such a configuration is useless,
		 * so let's keep the code simple.
		 */
		if (!vmx->nested.virtual_apic_page)
			return -EFAULT;
	}

	return 0;

...

Then nested_vmx_run can do the nested_vmx_failValid if the function returns
an error.

Paolo
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