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

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Message-ID: <YWW7zGWUpqXLXE/4@google.com>
Date:   Tue, 12 Oct 2021 16:46:04 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Atish Patra <Atish.Patra@....com>
Cc:     "kvm-riscv@...ts.infradead.org" <kvm-riscv@...ts.infradead.org>,
        "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
        "vincent.chen@...ive.com" <vincent.chen@...ive.com>,
        Anup Patel <Anup.Patel@....com>,
        "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 Mon, 2021-10-11 at 14:32 +0000, Sean Christopherson wrote:
> > 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?
> 
> Are you suggesting to factor the csr reset part to a different function?

More or less.  I'm mostly asking why putting the vCPU is necessary.

> > > > >  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.
> 
> I am assuming that you are suggesting to add this logic at the end of
> the kvm_arch_vcpu_create() instead of kvm_arch_vcpu_postcreate().
> 
> vcpu_idx is assigned after kvm_arch_vcpu_create() returns in the
> kvm_vm_ioctl_create_vcpu. kvm_arch_vcpu_postcreate() is the arch hookup
> after vcpu_idx is assigned.

Ah, it's the consumption of vcpu->vcpu_idx that's problematic.  Thanks!