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

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Message-ID: <53F30C0C.9040601@redhat.com>
Date:	Tue, 19 Aug 2014 10:34:20 +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 19/08/2014 10:30, Wanpeng Li ha scritto:
> +		if (vmx->nested.virtual_apic_page)
> +			nested_release_page(vmx->nested.virtual_apic_page);
> +		vmx->nested.virtual_apic_page =
> +		   nested_get_page(vcpu, vmcs12->virtual_apic_page_addr);
> +		if (!vmx->nested.virtual_apic_page)
> +			exec_control &=
> +				~CPU_BASED_TPR_SHADOW;
> +		else
> +			vmcs_write64(VIRTUAL_APIC_PAGE_ADDR,
> +				page_to_phys(vmx->nested.virtual_apic_page));
> +
> +		/*
> +		 * If CR8 load exits are enabled, CR8 store exits are enabled,
> +		 * and virtualize APIC access is disabled, the processor would
> +		 * never notice. Doing it unconditionally is not correct, but
> +		 * it is the simplest thing.
> +		 */
> +		if (!(exec_control & CPU_BASED_TPR_SHADOW) &&
> +			!((exec_control & CPU_BASED_CR8_LOAD_EXITING) &&
> +				(exec_control & CPU_BASED_CR8_STORE_EXITING)))
> +			nested_vmx_failValid(vcpu, VMXERR_ENTRY_INVALID_CONTROL_FIELD);
> +

You aren't checking "virtualize APIC access" here, but the comment
mentions it.

As the comment says, failing the entry unconditionally could be the
simplest thing, which means moving the nested_vmx_failValid call inside
the "if (!vmx->nested.virtual_apic_page)".

If you want to check all of CR8_LOAD/CR8_STORE/VIRTUALIZE_APIC_ACCESS,
please mention in the comment that failing the vm entry is _not_ what
the processor does but it's basically the only possibility we have.  In
that case, I would also place the "if" within the "if
(!vmx->nested.virtual_apic_page)": it also simplifies the condition
because you don't have to check CPU_BASED_TPR_SHADOW anymore.

You can send v5 with these changes, and I'll apply it for 3.18.  Thanks!

Paolo
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