linux-kernel - Re: [PATCH v2 03/31] KVM: x86: hyper-v: Handle HVCALL_FLUSH_VIRTUAL_ADDRESS

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Message-ID: <Yk8jp5ZoQO6X8aTG@google.com>
Date:   Thu, 7 Apr 2022 17:47:19 +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-kernel@...r.kernel.org
Subject: Re: [PATCH v2 03/31] KVM: x86: hyper-v: Handle
 HVCALL_FLUSH_VIRTUAL_ADDRESS_LIST{,EX} calls gently

On Thu, Apr 07, 2022, Sean Christopherson wrote:
> > @@ -1857,12 +1940,13 @@ static u64 kvm_hv_flush_tlb(struct kvm_vcpu *vcpu, struct kvm_hv_hcall *hc)
> >  	struct hv_tlb_flush_ex flush_ex;
> >  	struct hv_tlb_flush flush;
> >  	DECLARE_BITMAP(vcpu_mask, KVM_MAX_VCPUS);
> > +	u64 entries[KVM_HV_TLB_FLUSH_RING_SIZE - 2];
> 
> What's up with the -2?  And given the multitude of things going on in this code,
> I'd strongly prefer this be tlbflush_entries.
> 
> Actually, if you do:
> 
> 	u64 __tlbflush_entries[KVM_HV_TLB_FLUSH_RING_SIZE - 2];
> 	u64 *tlbflush_entries;

Looking at future patches, tlb_flush_entries is better for consistency (apply everywhere).

> and drop all_addr, the code to get entries can be
> 
> 	if (hc->code == HVCALL_FLUSH_VIRTUAL_ADDRESS_SPACE ||
> 	    hc->code == HVCALL_FLUSH_VIRTUAL_ADDRESS_SPACE_EX ||
> 	    hc->rep_cnt > ARRAY_SIZE(tlbflush_entries)) {
> 		tlbfluish_entries = NULL;
> 	} else {
> 		if (kvm_hv_get_tlbflush_entries(kvm, hc, __tlbflush_entries,
> 						consumed_xmm_halves, data_offset))
> 			return HV_STATUS_INVALID_HYPERCALL_INPUT;
> 		tlbfluish_entries = __tlbflush_entries;

Heh, fluish, because TLB entries are somewhat fluid?

> 	}
> 
> and the calls to queue flushes becomes
> 
> 			hv_tlb_flush_ring_enqueue(v, tlbflush_entries, hc->rep_cnt);
> 
> That way a bug will "just" be a NULL pointer dereference and not consumption of
> uninitialized data (though such a bug might be caught be caught by the compiler).