linux-kernel - Re: [RFC PATCH v5 039/104] KVM: x86/mmu: Disallow fast page fault on private GPA

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Message-ID: <aad44bd9-7f5b-58e6-523a-ce9df57bee30@redhat.com>
Date:   Tue, 5 Apr 2022 15:22:05 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     isaku.yamahata@...el.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     isaku.yamahata@...il.com, Jim Mattson <jmattson@...gle.com>,
        erdemaktas@...gle.com, Connor Kuehl <ckuehl@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>
Subject: Re: [RFC PATCH v5 039/104] KVM: x86/mmu: Disallow fast page fault on
 private GPA

On 3/4/22 20:48, isaku.yamahata@...el.com wrote:
> From: Isaku Yamahata <isaku.yamahata@...el.com>
> 
> TDX requires TDX SEAMCALL to operate Secure EPT instead of direct memory
> access and TDX SEAMCALL is heavy operation.  Fast page fault on private GPA
> doesn't make sense.  Disallow fast page fault on private GPA.
> 
> Signed-off-by: Isaku Yamahata <isaku.yamahata@...el.com>
> ---
>   arch/x86/kvm/mmu/mmu.c | 7 +++++++
>   1 file changed, 7 insertions(+)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index e9212394a530..d8c1505155b0 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -3185,6 +3185,13 @@ static int fast_page_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
>   	u64 *sptep = NULL;
>   	uint retry_count = 0;
>   
> +	/*
> +	 * TDX private mapping doesn't support fast page fault because the EPT
> +	 * entry needs TDX SEAMCALL. not direct memory access.

"the EPT entry is read/written with TDX SEAMCALLs instead of direct 
memory access".

Reviewed-by: Paolo Bonzini <pbonzini@...hat.com>

> +	 */
> +	if (kvm_is_private_gpa(vcpu->kvm, fault->addr))
> +		return ret;
> +
>   	if (!page_fault_can_be_fast(fault))
>   		return ret;
>