linux-kernel - Re: [PATCH v3 04/34] KVM: x86: hyper-v: Handle HVCALL_FLUSH_VIRTUAL_ADDRESS

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Date:   Mon, 16 May 2022 19:41:08 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Michael Kelley <mikelley@...rosoft.com>,
        Siddharth Chandrasekaran <sidcha@...zon.de>,
        linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 04/34] KVM: x86: hyper-v: Handle
 HVCALL_FLUSH_VIRTUAL_ADDRESS_LIST{,EX} calls gently

On Thu, Apr 14, 2022, Vitaly Kuznetsov wrote:
> @@ -1862,15 +1890,58 @@ void kvm_hv_vcpu_flush_tlb(struct kvm_vcpu *vcpu)
>  {
>  	struct kvm_vcpu_hv_tlb_flush_ring *tlb_flush_ring;
>  	struct kvm_vcpu_hv *hv_vcpu = to_hv_vcpu(vcpu);
> +	struct kvm_vcpu_hv_tlb_flush_entry *entry;
> +	int read_idx, write_idx;
> +	u64 address;
> +	u32 count;
> +	int i, j;
>  
> -	kvm_vcpu_flush_tlb_guest(vcpu);
> -
> -	if (!hv_vcpu)
> +	if (!tdp_enabled || !hv_vcpu) {
> +		kvm_vcpu_flush_tlb_guest(vcpu);
>  		return;
> +	}
>  
>  	tlb_flush_ring = &hv_vcpu->tlb_flush_ring;
>  
> -	tlb_flush_ring->read_idx = tlb_flush_ring->write_idx;
> +	/*
> +	 * TLB flush must be performed on the target vCPU so 'read_idx'
> +	 * (AKA 'tail') cannot change underneath, the compiler is free
> +	 * to re-read it.
> +	 */
> +	read_idx = tlb_flush_ring->read_idx;
> +
> +	/*
> +	 * 'write_idx' (AKA 'head') can be concurently updated by a different
> +	 * vCPU so we must be sure it's read once.
> +	 */
> +	write_idx = READ_ONCE(tlb_flush_ring->write_idx);
> +
> +	/* Pairs with smp_wmb() in hv_tlb_flush_ring_enqueue() */
> +	smp_rmb();
> +
> +	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;

Bumping tlb_flush is inconsistent with how KVM handles INVLPG, and could be wrong
if the ring is empty (might be impossible without a bug?).  And if my math is right,
or at least in the ballpark, tlb_flush will be incremented once regardless of whether
the loop flushed 1 page or 64k pages (completely full ring, full count on every one).

I'd prefer to either drop the stat adjustment entirely, or bump invlpg in the loop, e.g.

diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
index 56f06cf85282..5654c9d56289 100644
--- a/arch/x86/kvm/hyperv.c
+++ b/arch/x86/kvm/hyperv.c
@@ -1945,10 +1945,11 @@ void kvm_hv_vcpu_flush_tlb(struct kvm_vcpu *vcpu)
        for (i = read_idx; i != write_idx; i = (i + 1) % KVM_HV_TLB_FLUSH_RING_SIZE) {
                address = tlb_flush_ring->entries[i] & PAGE_MASK;
                count = (tlb_flush_ring->entries[i] & ~PAGE_MASK) + 1;
-               for (j = 0; j < count; j++)
+               for (j = 0; j < count; j++) {
                        static_call(kvm_x86_flush_tlb_gva)(vcpu, address + j * PAGE_SIZE);
+                       ++vcpu->stat.invlpg;
+               }
        }
-       ++vcpu->stat.tlb_flush;

 out_empty_ring:
        tlb_flush_ring->read_idx = write_idx;


> +	goto out_empty_ring;
> +
> +out_flush_all:
> +	kvm_vcpu_flush_tlb_guest(vcpu);
> +
> +out_empty_ring:
> +	tlb_flush_ring->read_idx = write_idx;
>  }
>