linux-kernel - Re: [RFC PATCH v2 11/19] RISC-V: KVM: Implement VMID allocator

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Date:   Mon, 5 Aug 2019 15:37:12 +0530
From:   Anup Patel <anup@...infault.org>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Anup Patel <Anup.Patel@....com>,
        Palmer Dabbelt <palmer@...ive.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Radim K <rkrcmar@...hat.com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Atish Patra <Atish.Patra@....com>,
        Alistair Francis <Alistair.Francis@....com>,
        Damien Le Moal <Damien.LeMoal@....com>,
        Christoph Hellwig <hch@...radead.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH v2 11/19] RISC-V: KVM: Implement VMID allocator

On Fri, Aug 2, 2019 at 2:49 PM Paolo Bonzini <pbonzini@...hat.com> wrote:
>
> On 02/08/19 09:48, Anup Patel wrote:
> > +struct kvm_vmid {
> > +     unsigned long vmid_version;
> > +     unsigned long vmid;
> > +};
> > +
>
> Please document that both fields are written under vmid_lock, and read
> outside it.

Sure, will add comments in asm/kvm_host.h

>
> > +             /*
> > +              * On SMP we know no other CPUs can use this CPU's or
> > +              * each other's VMID after forced exit returns since the
> > +              * vmid_lock blocks them from re-entry to the guest.
> > +              */
> > +             force_exit_and_guest_tlb_flush(cpu_all_mask);
>
> Please use kvm_flush_remote_tlbs(kvm) instead.  All you need to do to
> support it is check for KVM_REQ_TLB_FLUSH and handle it by calling
> __kvm_riscv_hfence_gvma_all.  Also, since your spinlock is global you
> probably should release it around the call to kvm_flush_remote_tlbs.
> (Think of an implementation that has a very small number of VMID bits).

Sure, I will use kvm_flush_remote_tlbs() here.

>
> > +     if (unlikely(vmid_next == 0)) {
> > +             WRITE_ONCE(vmid_version, READ_ONCE(vmid_version) + 1);
> > +             vmid_next = 1;
> > +             /*
> > +              * On SMP we know no other CPUs can use this CPU's or
> > +              * each other's VMID after forced exit returns since the
> > +              * vmid_lock blocks them from re-entry to the guest.
> > +              */
> > +             force_exit_and_guest_tlb_flush(cpu_all_mask);
> > +     }
> > +
> > +     vmid->vmid = vmid_next;
> > +     vmid_next++;
> > +     vmid_next &= (1 << vmid_bits) - 1;
> > +
> > +     /* Ensure VMID next update is completed */
> > +     smp_wmb();
>
> This barrier is not necessary.  Writes to vmid->vmid need not be ordered
> with writes to vmid->vmid_version, because the accesses happen in
> completely different places.

Yes, your right. There is already a WRITE_ONCE after it.
>
> (As a rule of thumb, each smp_wmb() should have a matching smp_rmb()
> somewhere, and this one doesn't).

Sure, thanks for the hint.

>
> Paolo
>
> > +     WRITE_ONCE(vmid->vmid_version, READ_ONCE(vmid_version));
> > +

Regards,
Anup