linux-kernel - Re: [PATCH 4.19 009/191] KVM: nVMX: Use correct root level for nested EPT shadow page tables

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Date:   Fri, 21 Feb 2020 07:05:12 -0800
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Pavel Machek <pavel@...x.de>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sasha Levin <sashal@...nel.org>
Subject: Re: [PATCH 4.19 009/191] KVM: nVMX: Use correct root level for
 nested EPT shadow page tables

On Fri, Feb 21, 2020 at 11:29:49AM +0100, Pavel Machek wrote:
> Hi!
> 
> > Hardcode the EPT page-walk level for L2 to be 4 levels, as KVM's MMU
> > currently also hardcodes the page walk level for nested EPT to be 4
> > levels.  The L2 guest is all but guaranteed to soft hang on its first
> > instruction when L1 is using EPT, as KVM will construct 4-level page
> > tables and then tell hardware to use 5-level page tables.
> 
> I don't get it. 7/191 reverts the patch, then 9/191 reverts the
> revert. Can we simply drop both 7 and 9, for exactly the same result?
>
> (Patch 8 is a unused file, so it does not change the picture).

Patch 07 is reverting this patch from the same unused file, 
arch/x86/kvm/vmx/vmx.c[*].  The reason patch 07 looks like a normal diff is
that a prior patch in 4.19.105 created the unused file (which is what's
reverted by patch 08 here).

Patch 09 reintroduces the fix for the correct file, arch/x86/kvm/vmx.c.

[*] In upstream, vmx.c now lives in arch/x86/kvm/vmx/, but in 4.19 and
    earlier it lives in arch/x86/kvm/.

> 
> Best regards,
> 								Pavel
> 
> > +++ b/arch/x86/kvm/vmx.c
> > @@ -5302,6 +5302,9 @@ static void vmx_set_cr0(struct kvm_vcpu *vcpu, unsigned long cr0)
> >  
> >  static int get_ept_level(struct kvm_vcpu *vcpu)
> >  {
> > +	/* Nested EPT currently only supports 4-level walks. */
> > +	if (is_guest_mode(vcpu) && nested_cpu_has_ept(get_vmcs12(vcpu)))
> > +		return 4;
> >  	if (cpu_has_vmx_ept_5levels() && (cpuid_maxphyaddr(vcpu) > 48))
> >  		return 5;
> >  	return 4;
> > -- 
> > 2.20.1
> > 
> > 
> 
> -- 
> (english) http://www.livejournal.com/~pavelmachek
> (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html