linux-kernel - Re: [PATCH 5/9] KVM: arm: move has_run_once after the map

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Date:   Thu, 14 Jan 2021 11:02:49 +0100
From:   Auger Eric <eric.auger@...hat.com>
To:     Alexandru Elisei <alexandru.elisei@....com>,
        eric.auger.pro@...il.com, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, kvmarm@...ts.cs.columbia.edu, maz@...nel.org,
        drjones@...hat.com
Cc:     james.morse@....com, julien.thierry.kdev@...il.com,
        suzuki.poulose@....com, shuah@...nel.org, pbonzini@...hat.com
Subject: Re: [PATCH 5/9] KVM: arm: move has_run_once after the map_resources

Hi Alexandru,

On 1/12/21 3:55 PM, Alexandru Elisei wrote:
> Hi Eric,
> 
> On 12/12/20 6:50 PM, Eric Auger wrote:
>> has_run_once is set to true at the beginning of
>> kvm_vcpu_first_run_init(). This generally is not an issue
>> except when exercising the code with KVM selftests. Indeed,
>> if kvm_vgic_map_resources() fails due to erroneous user settings,
>> has_run_once is set and this prevents from continuing
>> executing the test. This patch moves the assignment after the
>> kvm_vgic_map_resources().
>>
>> Signed-off-by: Eric Auger <eric.auger@...hat.com>
>> ---
>>  arch/arm64/kvm/arm.c | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
>> index c0ffb019ca8b..331fae6bff31 100644
>> --- a/arch/arm64/kvm/arm.c
>> +++ b/arch/arm64/kvm/arm.c
>> @@ -540,8 +540,6 @@ static int kvm_vcpu_first_run_init(struct kvm_vcpu *vcpu)
>>  	if (!kvm_arm_vcpu_is_finalized(vcpu))
>>  		return -EPERM;
>>  
>> -	vcpu->arch.has_run_once = true;
>> -
>>  	if (likely(irqchip_in_kernel(kvm))) {
>>  		/*
>>  		 * Map the VGIC hardware resources before running a vcpu the
>> @@ -560,6 +558,8 @@ static int kvm_vcpu_first_run_init(struct kvm_vcpu *vcpu)
>>  		static_branch_inc(&userspace_irqchip_in_use);
>>  	}
>>  
>> +	vcpu->arch.has_run_once = true;
> 
> I have a few concerns regarding this:
> 
> 1. Moving has_run_once = true here seems very arbitrary to me - kvm_timer_enable()
> and kvm_arm_pmu_v3_enable(), below it, can both fail because of erroneous user
> values. If there's a reason why the assignment cannot be moved at the end of the
> function, I think it should be clearly stated in a comment for the people who
> might be tempted to write similar tests for the timer or pmu.

Setting has_run_once = true at the entry of the function looks to me
even more arbitrary. I agree with you that eventually has_run_once may
be moved at the very end but maybe this can be done later once timer,
pmu tests haven ben written
> 
> 2. There are many ways that kvm_vgic_map_resources() can fail, other than
> incorrect user settings. I started digging into how
> kvm_vgic_map_resources()->vgic_v2_map_resources() can fail for a VGIC V2 and this
> is what I managed to find before I gave up:
> 
> * vgic_init() can fail in:
>     - kvm_vgic_dist_init()
>     - vgic_v3_init()
>     - kvm_vgic_setup_default_irq_routing()
> * vgic_register_dist_iodev() can fail in:
>     - vgic_v3_init_dist_iodev()
>     - kvm_io_bus_register_dev()(*)
> * kvm_phys_addr_ioremap() can fail in:
>     - kvm_mmu_topup_memory_cache()
>     - kvm_pgtable_stage2_map()

I changed the commit msg so that "incorrect user settings" sounds as an
example.
> 
> So if any of the functions below fail, are we 100% sure it is safe to allow the
> user to execute kvm_vgic_map_resources() again?

I think additional tests will confirm this. However at the moment,
moving the assignment, which does not look wrong to me, allows to
greatly simplify the tests so I would tend to say that it is worth.
> 
> (*) It looks to me like kvm_io_bus_register_dev() doesn't take into account a
> caller that tries to register the same device address range and it will create
> another identical range. Is this intentional? Is it a bug that should be fixed? Or
> am I misunderstanding the function?

doesn't kvm_io_bus_cmp() do the check?

Thanks

Eric
> 
> Thanks,
> Alex
>> +
>>  	ret = kvm_timer_enable(vcpu);
>>  	if (ret)
>>  		return ret;
>