linux-kernel - Re: [PATCH v2 26/43] KVM: VMX: Read Posted Interrupt "control" exactly once per loop iteration

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Message-ID: <b078cce30f86672d7d8f8eaa0adc47d24def24e2.camel@redhat.com>
Date:   Thu, 28 Oct 2021 13:58:40 +0300
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Marc Zyngier <maz@...nel.org>,
        Huacai Chen <chenhuacai@...nel.org>,
        Aleksandar Markovic <aleksandar.qemu.devel@...il.com>,
        Paul Mackerras <paulus@...abs.org>,
        Anup Patel <anup.patel@....com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Atish Patra <atish.patra@....com>,
        David Hildenbrand <david@...hat.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-mips@...r.kernel.org, kvm@...r.kernel.org,
        kvm-ppc@...r.kernel.org, kvm-riscv@...ts.infradead.org,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        David Matlack <dmatlack@...gle.com>,
        Oliver Upton <oupton@...gle.com>,
        Jing Zhang <jingzhangos@...gle.com>
Subject: Re: [PATCH v2 26/43] KVM: VMX: Read Posted Interrupt "control"
 exactly once per loop iteration

On Fri, 2021-10-08 at 19:12 -0700, Sean Christopherson wrote:
> Use READ_ONCE() when loading the posted interrupt descriptor control
> field to ensure "old" and "new" have the same base value.  If the
> compiler emits separate loads, and loads into "new" before "old", KVM
> could theoretically drop the ON bit if it were set between the loads.
> 
> Fixes: 28b835d60fcc ("KVM: Update Posted-Interrupts Descriptor when vCPU is preempted")
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  arch/x86/kvm/vmx/posted_intr.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/posted_intr.c b/arch/x86/kvm/vmx/posted_intr.c
> index 414ea6972b5c..fea343dcc011 100644
> --- a/arch/x86/kvm/vmx/posted_intr.c
> +++ b/arch/x86/kvm/vmx/posted_intr.c
> @@ -53,7 +53,7 @@ void vmx_vcpu_pi_load(struct kvm_vcpu *vcpu, int cpu)
>  
>  	/* The full case.  */
>  	do {
> -		old.control = new.control = pi_desc->control;
> +		old.control = new.control = READ_ONCE(pi_desc->control);
>  
>  		dest = cpu_physical_id(cpu);
>  
> @@ -104,7 +104,7 @@ static void __pi_post_block(struct kvm_vcpu *vcpu)
>  	     "Wakeup handler not enabled while the vCPU was blocking");
>  
>  	do {
> -		old.control = new.control = pi_desc->control;
> +		old.control = new.control = READ_ONCE(pi_desc->control);
>  
>  		dest = cpu_physical_id(vcpu->cpu);
>  
> @@ -160,7 +160,7 @@ int pi_pre_block(struct kvm_vcpu *vcpu)
>  	     "Posted Interrupt Suppress Notification set before blocking");
>  
>  	do {
> -		old.control = new.control = pi_desc->control;
> +		old.control = new.control = READ_ONCE(pi_desc->control);
>  
>  		/* set 'NV' to 'wakeup vector' */
>  		new.nv = POSTED_INTR_WAKEUP_VECTOR;

I wish there was a way to mark fields in a struct, as requiring 'READ_ONCE' on them
so that compiler would complain if this isn't done, or automatically use 'READ_ONCE'
logic.

Reviewed-by: Maxim Levitsky <mlevitsk@...hat.com>

Best regards,
	Maxim Levitsky