linux-kernel - Re: [PATCH] KVM: x86: Mark CR4.UMIP as reserved based on associated CPUID bit

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Message-ID: <87a7669u6q.fsf@vitty.brq.redhat.com>
Date:   Wed, 29 Jan 2020 16:08:29 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH] KVM: x86: Mark CR4.UMIP as reserved based on associated CPUID bit

Sean Christopherson <sean.j.christopherson@...el.com> writes:

> Re-add code to mark CR4.UMIP as reserved if UMIP is not supported by the
> host.  The UMIP handling was unintentionally dropped during a recent
> refactoring.
>
> Not flagging CR4.UMIP allows the guest to set its CR4.UMIP regardless of
> host support or userspace desires.  On CPUs with UMIP support, including
> emulated UMIP, this allows the guest to enable UMIP against the wishes
> of the userspace VMM.  On CPUs without any form of UMIP, this results in
> a failed VM-Enter due to invalid guest state.
>
> Fixes: 345599f9a2928 ("KVM: x86: Add macro to ensure reserved cr4 bits checks stay in sync")

This one in only in kvm/next so just in case another force-push is
around the corner I'd suggest we squash this change in :-)

> Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
> ---
>  arch/x86/kvm/x86.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 7e3f1d937224..e70d1215638a 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -898,6 +898,8 @@ EXPORT_SYMBOL_GPL(kvm_set_xcr);
>  		__reserved_bits |= X86_CR4_PKE;		\
>  	if (!__cpu_has(__c, X86_FEATURE_LA57))		\
>  		__reserved_bits |= X86_CR4_LA57;	\
> +	if (!__cpu_has(__c, X86_FEATURE_UMIP))		\
> +		__reserved_bits |= X86_CR4_UMIP;	\
>  	__reserved_bits;				\
>  })

Reviewed-by: Vitaly Kuznetsov <vkuznets@...hat.com>

-- 
Vitaly