linux-kernel - Re: [PATCH v12 13/46] KVM: x86: Prepare kvm_hv_flush

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Message-ID: <Y1r6do4AVmAVvxeI@google.com>
Date:   Thu, 27 Oct 2022 21:39:02 +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>,
        Yuan Yao <yuan.yao@...ux.intel.com>,
        Maxim Levitsky <mlevitsk@...hat.com>,
        linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v12 13/46] KVM: x86: Prepare kvm_hv_flush_tlb() to handle
 L2's GPAs

On Thu, Oct 27, 2022, Vitaly Kuznetsov wrote:
> Sean Christopherson <seanjc@...gle.com> writes:
> 
> > On Fri, Oct 21, 2022, Vitaly Kuznetsov wrote:
> >> @@ -1908,6 +1909,12 @@ static u64 kvm_hv_flush_tlb(struct kvm_vcpu *vcpu, struct kvm_hv_hcall *hc)
> >>  	 */
> >>  	BUILD_BUG_ON(KVM_HV_MAX_SPARSE_VCPU_SET_BITS > 64);
> >>  
> >> +	if (!hc->fast && is_guest_mode(vcpu)) {
> >
> > Please add a comment explaining why only "slow" hypercalls need to translate the
> > GPA from L2=>L1.
> >
> > With a comment (and assuming this isn't a bug),
> 
> This is intended,
> 
> For "slow" hypercalls 'hc->ingpa' is the GPA (or an 'nGPA' -- thus the
> patch) in guest memory where hypercall parameters are placed, kvm reads
> them with kvm_read_guest() later. For "fast" hypercalls 'ingpa' is a
> misnomer as it is not an address but the first parameter (in the 'tlb
> flush' case it's 'address space id' which we currently don't
> analyze). We may want to add a union in 'struct kvm_hv_hcall' to make
> this explicit.

Ya, a union would be helpful.  I'm pretty sure at some point I knew the "fast"
ingpa isn't actually a GPA, but obviously forgot that detail.