Re: [Part2 PATCH v5 31/31] KVM: X86: Restart the guest when insn_len is zero and SEV is enabled

[Paolo Bonzini]

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