Re: [PATCH v2 03/31] KVM: x86: hyper-v: Handle HVCALL_FLUSH_VIRTUAL_ADDRESS_LIST{,EX} calls gently

[Sean Christopherson <seanjc@google.com>]

On Mon, Apr 11, 2022, Vitaly Kuznetsov wrote:
> Sean Christopherson <seanjc@google.com> writes:
> 
> > On Thu, Apr 07, 2022, Vitaly Kuznetsov wrote:
> >> @@ -1840,15 +1891,47 @@ void kvm_hv_vcpu_flush_tlb(struct kvm_vcpu *vcpu)
> >>  {
> >>  	struct kvm_vcpu_hv_tlbflush_ring *tlb_flush_ring;
> >>  	struct kvm_vcpu_hv *hv_vcpu = to_hv_vcpu(vcpu);
> >> -
> >> -	kvm_vcpu_flush_tlb_guest(vcpu);
> >> -
> >> -	if (!hv_vcpu)
> >> +	struct kvm_vcpu_hv_tlbflush_entry *entry;
> >> +	int read_idx, write_idx;
> >> +	u64 address;
> >> +	u32 count;
> >> +	int i, j;
> >> +
> >> +	if (!tdp_enabled || !hv_vcpu) {
> >> +		kvm_vcpu_flush_tlb_guest(vcpu);
> >>  		return;
> >> +	}
> >>  
> >>  	tlb_flush_ring = &hv_vcpu->tlb_flush_ring;
> >> +	read_idx = READ_ONCE(tlb_flush_ring->read_idx);
> >> +	write_idx = READ_ONCE(tlb_flush_ring->write_idx);
> >> +
> >> +	/* Pairs with smp_wmb() in hv_tlb_flush_ring_enqueue() */
> >> +	smp_rmb();
> >>  
> >> -	tlb_flush_ring->read_idx = tlb_flush_ring->write_idx;
> >> +	for (i = read_idx; i != write_idx; i = (i + 1) % KVM_HV_TLB_FLUSH_RING_SIZE) {
> >> +		entry = &tlb_flush_ring->entries[i];
> >> +
> >> +		if (entry->flush_all)
> >> +			goto out_flush_all;
> >> +
> >> +		/*
> >> +		 * Lower 12 bits of 'address' encode the number of additional
> >> +		 * pages to flush.
> >> +		 */
> >> +		address = entry->addr & PAGE_MASK;
> >> +		count = (entry->addr & ~PAGE_MASK) + 1;
> >> +		for (j = 0; j < count; j++)
> >> +			static_call(kvm_x86_flush_tlb_gva)(vcpu, address + j * PAGE_SIZE);
> >> +	}
> >> +	++vcpu->stat.tlb_flush;
> >> +	goto out_empty_ring;
> >> +
> >> +out_flush_all:
> >> +	kvm_vcpu_flush_tlb_guest(vcpu);
> >> +
> >> +out_empty_ring:
> >> +	tlb_flush_ring->read_idx = write_idx;
> >
> > Does this need WRITE_ONCE?  My usual "I suck at memory ordering" disclaimer applies.
> >
> 
> Same here) I *think* we're fine for 'read_idx' as it shouldn't matter at
> which point in this function 'tlb_flush_ring->read_idx' gets modified
> (relative to other things, e.g. actual TLB flushes) and there's no
> concurency as we only have one reader (the vCPU which needs its TLB
> flushed). On the other hand, I'm not against adding WRITE_ONCE() here
> even if just to aid an unprepared reader (thinking myself couple years
> in the future).

Ah, read_idx == tail and write_idx == head.  I didn't look at the structure very
closely, or maybe not at all :-)  And IIUC, only the vCPU itself ever writes to
tail?  In that case, I would omit the READ_ONCE() from both the write to tail here
and the read above, and probably add a brief comment stating that the flush must
be performed on the target vCPU, i.e. must hold vcpu->mutex, and so it's safe for
the compiler to re-read tlb_flush_ring->read_idx in the loop because it cannot
change.