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    SubjectRe: [RFC PATCH v2 11/19] RISC-V: KVM: Implement VMID allocator
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    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.

    > + /*
    > + * 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).

    > + 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.

    (As a rule of thumb, each smp_wmb() should have a matching smp_rmb()
    somewhere, and this one doesn't).

    Paolo

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

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