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

[Paolo Bonzini]

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?

Thanks,

Paolo