linux-kernel - Re: [PATCH 2/2] KVM: X86: XCR0 should be set to the fixed value on vCPU reset

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Date:   Mon, 9 Oct 2017 10:04:55 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     Radim Krčmář <rkrcmar@...hat.com>,
        Wanpeng Li <wanpeng.li@...mail.com>
Subject: Re: [PATCH 2/2] KVM: X86: XCR0 should be set to the fixed value on
 vCPU reset

On 08/10/2017 04:35, Wanpeng Li wrote:
> From: Wanpeng Li <wanpeng.li@...mail.com>
> 
> SDM section 2.6 mentioned: 
> 
> After reset, all bits (except bit 0) in XCR0 are cleared to zero; XCR0[0] is set to 1.
> 
> This patch sets XCRO to the 0x1 after vCPU reset.
> 
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: Radim Krčmář <rkrcmar@...hat.com>
> Signed-off-by: Wanpeng Li <wanpeng.li@...mail.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 b0d2915..c784cd6 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -7816,6 +7816,8 @@ void kvm_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
>  	vcpu->arch.regs_avail = ~0;
>  	vcpu->arch.regs_dirty = ~0;
>  
> +	vcpu->arch.xcr0 = XFEATURE_MASK_FP;

XCR0 is unchanged by INIT.  There are many other registers that are
zeroed by RESET by not INIT; KVM pretty much relies on userspace doing
that.  On the other hand, it's useful to zero here those registers that
are zeroed by INIT as well:

- XSS

- BNDCFGU, BND0-BND3, BNDCFGS (BNDCFGS probably would go in
vmx_vcpu_reset, the others are in vcpu->arch.guest_fpu.state.xsave).

>  	kvm_x86_ops->vcpu_reset(vcpu, init_event);
>  }
>  
>