linux-kernel - Re: [PATCH v3 5/5] RISC-V: Add SBI HSM extension in KVM

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Date:   Mon, 11 Oct 2021 14:32:38 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Atish Patra <Atish.Patra@....com>
Cc:     "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
        Anup Patel <Anup.Patel@....com>,
        "vincent.chen@...ive.com" <vincent.chen@...ive.com>,
        "kvm-riscv@...ts.infradead.org" <kvm-riscv@...ts.infradead.org>,
        "paul.walmsley@...ive.com" <paul.walmsley@...ive.com>,
        "palmer@...belt.com" <palmer@...belt.com>,
        "wangkefeng.wang@...wei.com" <wangkefeng.wang@...wei.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 5/5] RISC-V: Add SBI HSM extension in KVM

On Mon, Oct 11, 2021, Atish Patra wrote:
> On Fri, 2021-10-08 at 15:02 +0000, Sean Christopherson wrote:
> > On Thu, Oct 07, 2021, Atish Patra wrote:
> > > +       preempt_disable();
> > > +       loaded = (vcpu->cpu != -1);
> > > +       if (loaded)
> > > +               kvm_arch_vcpu_put(vcpu);
> > 
> > Oof.  Looks like this pattern was taken from arm64. 
> 
> Yes. This part is similar to arm64 because the same race condition can
> happen in riscv due to save/restore of CSRs during reset.
> 
> 
> >  Is there really no better approach to handling this?  I don't see anything
> >  in kvm_riscv_reset_vcpu() that will obviously break if the vCPU is
> >  loaded.  If the goal is purely to effect a CSR reset via
> >  kvm_arch_vcpu_load(), then why not just factor out a helper to do exactly
> >  that?

What about the question here?

> > 
> > >  
> > >         memcpy(csr, reset_csr, sizeof(*csr));
> > >  
> > > @@ -144,6 +151,11 @@ static void kvm_riscv_reset_vcpu(struct
> > > kvm_vcpu *vcpu)
> > >  
> > >         WRITE_ONCE(vcpu->arch.irqs_pending, 0);
> > >         WRITE_ONCE(vcpu->arch.irqs_pending_mask, 0);
> > > +
> > > +       /* Reset the guest CSRs for hotplug usecase */
> > > +       if (loaded)
> > > +               kvm_arch_vcpu_load(vcpu, smp_processor_id());
> > 
> > If the preempt shenanigans really have to stay, at least use
> > get_cpu()/put_cpu().
> > 
> 
> Is there any specific advantage to that ? get_cpu/put_cpu are just
> macros which calls preempt_disable/preempt_enable.
> 
> The only advantage of get_cpu is that it returns the current cpu. 
> vcpu_load function uses get_cpu because it requires the current cpu id.
> 
> However, we don't need that in this case. I am not against changing it
> to get_cpu/put_cpu. Just wanted to understand the reasoning behind your
> suggestion.

It would make the code a bit self-documenting, because AFAICT it doesn't truly
care about being preempted, it cares about keeping the vCPU on the correct pCPU.

> > > +       preempt_enable();
> > >  }
> > >  
> > >  int kvm_arch_vcpu_precreate(struct kvm *kvm, unsigned int id)
> > > @@ -180,6 +192,13 @@ int kvm_arch_vcpu_create(struct kvm_vcpu
> > > *vcpu)
> > >  
> > >  void kvm_arch_vcpu_postcreate(struct kvm_vcpu *vcpu)
> > >  {
> > > +       /**
> > > +        * vcpu with id 0 is the designated boot cpu.
> > > +        * Keep all vcpus with non-zero cpu id in power-off state
> > > so that they
> > > +        * can brought to online using SBI HSM extension.
> > > +        */
> > > +       if (vcpu->vcpu_idx != 0)
> > > +               kvm_riscv_vcpu_power_off(vcpu);
> > 
> > Why do this in postcreate?
> > 
> 
> Because we need to absolutely sure that the vcpu is created. It is
> cleaner in this way rather than doing this here at the end of
> kvm_arch_vcpu_create. create_vcpu can also fail after
> kvm_arch_vcpu_create returns.

But kvm_riscv_vcpu_power_off() doesn't doesn't anything outside of the vCPU.  It
clears vcpu->arch.power_off, makes a request, and kicks the vCPU.  None of that
has side effects to anything else in KVM.  If the vCPU isn't created successfully,
it gets deleted and nothing ever sees that state change.