linux-kernel - Re: [RFC PATCH 09/35] KVM: SVM: Do not emulate MMIO under SEV-ES

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Date:   Tue, 15 Sep 2020 08:38:52 -0500
From:   Tom Lendacky <thomas.lendacky@....com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org, x86@...nel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Brijesh Singh <brijesh.singh@....com>
Subject: Re: [RFC PATCH 09/35] KVM: SVM: Do not emulate MMIO under SEV-ES

On 9/14/20 4:33 PM, Sean Christopherson wrote:
> On Mon, Sep 14, 2020 at 03:15:23PM -0500, Tom Lendacky wrote:
>> From: Tom Lendacky <thomas.lendacky@....com>
>>
>> When a guest is running as an SEV-ES guest, it is not possible to emulate
>> MMIO. Add support to prevent trying to perform MMIO emulation.
>>
>> Signed-off-by: Tom Lendacky <thomas.lendacky@....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 a5d0207e7189..2e1b8b876286 100644
>> --- a/arch/x86/kvm/mmu/mmu.c
>> +++ b/arch/x86/kvm/mmu/mmu.c
>> @@ -5485,6 +5485,13 @@ int kvm_mmu_page_fault(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa, u64 error_code,
>>  	if (!mmio_info_in_cache(vcpu, cr2_or_gpa, direct) && !is_guest_mode(vcpu))
>>  		emulation_type |= EMULTYPE_ALLOW_RETRY_PF;
>>  emulate:
>> +	/*
>> +	 * When the guest is an SEV-ES guest, emulation is not possible.  Allow
>> +	 * the guest to handle the MMIO emulation.
>> +	 */
>> +	if (vcpu->arch.vmsa_encrypted)
>> +		return 1;
> 
> A better approach is to refactor need_emulation_on_page_fault() (the hook
> that's just out of sight in this patch) into a more generic
> kvm_x86_ops.is_emulatable() so that the latter can be used to kill emulation
> everywhere, and for other reasons.  E.g. TDX obviously shares very similar
> logic, but SGX also adds a case where KVM can theoretically end up in an
> emulator path without the ability to access the necessary guest state.
> 
> I have exactly such a prep patch (because SGX and TDX...), I'll get it posted
> in the next day or two.

Sounds good. I'll check it out when it's posted.

Thanks,
Tom

> 
>> +
>>  	/*
>>  	 * On AMD platforms, under certain conditions insn_len may be zero on #NPF.
>>  	 * This can happen if a guest gets a page-fault on data access but the HW
>> -- 
>> 2.28.0
>>