linux-kernel - Re: [PATCH] KVM: x86: Don't attempt to load PDPTRs when 64-bit mode is enabled

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Message-ID: <87wo36s3wb.fsf@vitty.brq.redhat.com>
Date:   Tue, 14 Jul 2020 14:00:04 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Oliver Upton <oupton@...gle.com>,
        Peter Shier <pshier@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH] KVM: x86: Don't attempt to load PDPTRs when 64-bit mode is enabled

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

> Don't attempt to load PDPTRs if EFER.LME=1, i.e. if 64-bit mode is
> enabled.  A recent change to reload the PDTPRs when CR0.CD or CR0.NW is
> toggled botched the EFER.LME handling and sends KVM down the PDTPR path
> when is_paging() is true, i.e. when the guest toggles CD/NW in 64-bit
> mode.
>
> Split the CR0 checks for 64-bit vs. 32-bit PAE into separate paths.  The
> 64-bit path is specifically checking state when paging is toggled on,
> i.e. CR0.PG transititions from 0->1.  The PDPTR path now needs to run if
> the new CR0 state has paging enabled, irrespective of whether paging was
> already enabled.  Trying to shave a few cycles to make the PDPTR path an
> "else if" case is a mess.
>
> Fixes: d42e3fae6faed ("kvm: x86: Read PDPTEs on CR0.CD and CR0.NW changes")
> Cc: Jim Mattson <jmattson@...gle.com>
> Cc: Oliver Upton <oupton@...gle.com>
> Cc: Peter Shier <pshier@...gle.com>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
> ---
>
> The other way to fix this, with a much smaller diff stat, is to simply
> move the !is_page(vcpu) check inside (vcpu->arch.efer & EFER_LME).  But
> that results in a ridiculous amount of nested conditionals for what is a
> very straightforward check e.g.
>
> 	if (cr0 & X86_CR0_PG) {
> 		if (vcpu->arch.efer & EFER_LME) }
> 			if (!is_paging(vcpu)) {
> 				...
> 			}
> 		}
> 	}
>
> Since this doesn't need to be backported anywhere, I didn't see any value
> in having an intermediate step.
>
>  arch/x86/kvm/x86.c | 24 ++++++++++++------------
>  1 file changed, 12 insertions(+), 12 deletions(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 95ef629228691..5f526d94c33f3 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -819,22 +819,22 @@ int kvm_set_cr0(struct kvm_vcpu *vcpu, unsigned long cr0)
>  	if ((cr0 & X86_CR0_PG) && !(cr0 & X86_CR0_PE))
>  		return 1;
>  
> -	if (cr0 & X86_CR0_PG) {
>  #ifdef CONFIG_X86_64
> -		if (!is_paging(vcpu) && (vcpu->arch.efer & EFER_LME)) {
> -			int cs_db, cs_l;
> +	if ((vcpu->arch.efer & EFER_LME) && !is_paging(vcpu) &&
> +	    (cr0 & X86_CR0_PG)) {

it seems we have more than one occurance of "if (vcpu->arch.efer &
EFER_LME)" under "#ifdef CONFIG_X86_64" and we alredy have 

static inline int is_long_mode(struct kvm_vcpu *vcpu)
{
#ifdef CONFIG_X86_64
     return vcpu->arch.efer & EFER_LMA;
#else
     return 0;
#endif
}

so if we use this instead, the compilers will just throw away the
non-reachable blocks when !(#ifdef CONFIG_X86_64), right?

> +		int cs_db, cs_l;
>  
> -			if (!is_pae(vcpu))
> -				return 1;
> -			kvm_x86_ops.get_cs_db_l_bits(vcpu, &cs_db, &cs_l);
> -			if (cs_l)
> -				return 1;
> -		} else
> -#endif
> -		if (is_pae(vcpu) && ((cr0 ^ old_cr0) & pdptr_bits) &&
> -		    !load_pdptrs(vcpu, vcpu->arch.walk_mmu, kvm_read_cr3(vcpu)))
> +		if (!is_pae(vcpu))
> +			return 1;
> +		kvm_x86_ops.get_cs_db_l_bits(vcpu, &cs_db, &cs_l);
> +		if (cs_l)
>  			return 1;
>  	}
> +#endif
> +	if (!(vcpu->arch.efer & EFER_LME) && (cr0 & X86_CR0_PG) &&
> +	    is_pae(vcpu) && ((cr0 ^ old_cr0) & pdptr_bits) &&
> +	    !load_pdptrs(vcpu, vcpu->arch.walk_mmu, kvm_read_cr3(vcpu)))
> +		return 1;
>  
>  	if (!(cr0 & X86_CR0_PG) && kvm_read_cr4_bits(vcpu, X86_CR4_PCIDE))
>  		return 1;

-- 
Vitaly