linux-kernel - Re: [PATCH] KVM: VMX: Set VMENTER_L1D_FLUSH_NOT_REQUIRED if !X86_BUG

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Message-ID: <20190927155518.GB23002@zn.tnic>
Date:   Fri, 27 Sep 2019 17:55:18 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Waiman Long <longman@...hat.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: VMX: Set VMENTER_L1D_FLUSH_NOT_REQUIRED if
 !X86_BUG_L1TF

On Thu, Sep 26, 2019 at 01:29:28PM -0400, Waiman Long wrote:
> On 8/26/19 3:30 PM, Waiman Long wrote:
> > The l1tf_vmx_mitigation is only set to VMENTER_L1D_FLUSH_NOT_REQUIRED
> > when the ARCH_CAPABILITIES MSR indicates that L1D flush is not required.
> > However, if the CPU is not affected by L1TF, l1tf_vmx_mitigation will
> > still be set to VMENTER_L1D_FLUSH_AUTO. This is certainly not the best
> > option for a !X86_BUG_L1TF CPU.
> >
> > So force l1tf_vmx_mitigation to VMENTER_L1D_FLUSH_NOT_REQUIRED to make it
> > more explicit in case users are checking the vmentry_l1d_flush parameter.
> >
> > Signed-off-by: Waiman Long <longman@...hat.com>
> > ---
> >  arch/x86/kvm/vmx/vmx.c | 2 ++
> >  1 file changed, 2 insertions(+)
> >
> > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> > index 42ed3faa6af8..a00ce3d6bbfd 100644
> > --- a/arch/x86/kvm/vmx/vmx.c
> > +++ b/arch/x86/kvm/vmx/vmx.c
> > @@ -7896,6 +7896,8 @@ static int __init vmx_init(void)
> >  			vmx_exit();
> >  			return r;
> >  		}
> > +	} else {
> > +		l1tf_vmx_mitigation = VMENTER_L1D_FLUSH_NOT_REQUIRED;
> >  	}
> >  
> >  #ifdef CONFIG_KEXEC_CORE
> 
> Ping. Any comment on that one?

I'd move that logic with the if (boot_cpu_has(X86_BUG_L1TF)) check inside
vmx_setup_l1d_flush() so that I have this:

        if (!boot_cpu_has_bug(X86_BUG_L1TF)) {
                l1tf_vmx_mitigation = VMENTER_L1D_FLUSH_NOT_REQUIRED;
                return 0;
        }

	if (!enable_ept) {
		...

	}

inside the function and outside am left with:

	r = vmx_setup_l1d_flush(vmentry_l1d_flush_param);
        if (r) {
		vmx_exit();
                return r;
	}

only. This way I'm concentrating the whole l1tf_vmx_mitigation picking
apart in one place.

Also, note that X86_BUG flags are checked with boot_cpu_has_bug() even
if it boils down to the same thing now.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette