Re: [PATCH 05/11] KVM: arm64: Start handling SMCs from EL1

From: Marc Zyngier <maz@kernel.org>
To: Suzuki K Poulose <suzuki.poulose@arm.com>
Cc: Oliver Upton <oliver.upton@linux.dev>,
	kvmarm@lists.linux.dev, kvm@vger.kernel.org,
	Paolo Bonzini <pbonzini@redhat.com>,
	James Morse <james.morse@arm.com>,
	Zenghui Yu <yuzenghui@huawei.com>,
	Sean Christopherson <seanjc@google.com>,
	Salil Mehta <salil.mehta@huawei.com>
Subject: Re: [PATCH 05/11] KVM: arm64: Start handling SMCs from EL1
Date: Tue, 28 Mar 2023 15:15:23 +0100	[thread overview]
Message-ID: <3712e4370dafe95ab828d642dbf064f8@kernel.org> (raw)
In-Reply-To: <23758eb0-a5b9-afa6-a85e-faa2690323c7@arm.com>

On 2023-03-28 09:52, Suzuki K Poulose wrote:
> On 20/03/2023 22:09, Oliver Upton wrote:
>> Whelp, the architecture gods have spoken and confirmed that the 
>> function
>> ID space is common between SMCs and HVCs. Not only that, the 
>> expectation
>> is that hypervisors handle calls to both SMC and HVC conduits. KVM
>> recently picked up support for SMCCCs in commit bd36b1a9eb5a ("KVM:
>> arm64: nv: Handle SMCs taken from virtual EL2") but scoped it only to 
>> a
>> nested hypervisor.
>> 
>> Let's just open the floodgates and let EL1 access our SMCCC
>> implementation with the SMC instruction as well.
>> 
>> Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
> 
> Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
> 
> One minor observation below.
> 
>> ---
>>   arch/arm64/kvm/handle_exit.c | 14 +++++++-------
>>   1 file changed, 7 insertions(+), 7 deletions(-)
>> 
>> diff --git a/arch/arm64/kvm/handle_exit.c 
>> b/arch/arm64/kvm/handle_exit.c
>> index 5e4f9737cbd5..68f95dcd41a1 100644
>> --- a/arch/arm64/kvm/handle_exit.c
>> +++ b/arch/arm64/kvm/handle_exit.c
>> @@ -72,13 +72,15 @@ static int handle_smc(struct kvm_vcpu *vcpu)
>>   	 *
>>   	 * We need to advance the PC after the trap, as it would
>>   	 * otherwise return to the same address...
>> -	 *
>> -	 * Only handle SMCs from the virtual EL2 with an immediate of zero 
>> and
>> -	 * skip it otherwise.
>>   	 */
>> -	if (!vcpu_is_el2(vcpu) || kvm_vcpu_hvc_get_imm(vcpu)) {
>> +	kvm_incr_pc(vcpu);
>> +
>> +	/*
>> +	 * SMCs with a nonzero immediate are reserved according to DEN0028E 
>> 2.9
>> +	 * "SMC and HVC immediate value".
>> +	 */
>> +	if (kvm_vcpu_hvc_get_imm(vcpu)) {
>>   		vcpu_set_reg(vcpu, 0, ~0UL);
>> -		kvm_incr_pc(vcpu);
>>   		return 1;
>>   	}
>>   @@ -93,8 +95,6 @@ static int handle_smc(struct kvm_vcpu *vcpu)
>>   	if (ret < 0)
>>   		vcpu_set_reg(vcpu, 0, ~0UL);
> 
> Nothing to do with this patch. But that check above is different
> from how we handle HVC. i.e., we return back to guest for HVCs.
> But for SMCs, we tend to return "ret" indicating an error (ret < 0).
> 
> Do we need to fix that ?

I guess so. It is just that it is practically impossible to get
a negative value at the moment, but it isn't something we should
rely on.

Thanks,

         M.
-- 
Jazz is not dead. It just smells funny...