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

[Sean Christopherson <seanjc@google.com>]

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.