linux-kernel - Re: [Part2 PATCH v5 31/31] KVM: X86: Restart the guest when insn

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Date:   Wed, 18 Oct 2017 11:31:43 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Brijesh Singh <brijesh.singh@....com>, x86@...nel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Joerg Roedel <joro@...tes.org>, Borislav Petkov <bp@...e.de>,
        Tom Lendacky <thomas.lendacky@....com>
Subject: Re: [Part2 PATCH v5 31/31] KVM: X86: Restart the guest when insn_len
 is zero and SEV is enabled

On 18/10/2017 11:26, Paolo Bonzini wrote:
> On 04/10/2017 15:17, Brijesh Singh wrote:
>> +	/*
>> +	 * 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
>> +	 * table walker is not able to read the instruction page (e.g instruction
>> +	 * page is not present in memory).
>> +	 *
>> +	 * Typically, when insn_len is zero, x86_emulate_instruction() walks the
>> +	 * guest page table and fetches the instruction bytes from guest memory.
>> +	 * When SEV is enabled, the guest memory is encrypted with guest-specific
>> +	 * key hence hypervisor will not able to fetch the instruction bytes.
>> +	 * In those cases we simply restart the guest.
>> +	 */
>> +	if (unlikely(!insn_len) &&
>> +	    kvm_x86_ops->mem_enc_enabled &&
>> +	    kvm_x86_ops->mem_enc_enabled(vcpu))
>> +		return 1;
>> +
> 
> Is it needed to test mem_enc_enabled?  Could it instead test for the
> availability of decode assists?

More precisely, you could test "unlikely(insn && !insn_len)" here and,
in svm.c, pass insn as

	static_cpu_has(X86_FEATURE_DECODEASSISTS) ?
                        svm->vmcb->control.insn_bytes : 0

Paolo