linux-kernel - Re: [PATCH v10 02/39] KVM: x86: hyper-v: Resurrect dedicated KVM_REQ_HV_TLB

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Date:   Thu, 22 Sep 2022 11:35:08 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Sean Christopherson <seanjc@...gle.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 v10 02/39] KVM: x86: hyper-v: Resurrect dedicated
 KVM_REQ_HV_TLB_FLUSH flag

Sean Christopherson <seanjc@...gle.com> writes:

> On Wed, Sep 21, 2022, Sean Christopherson wrote:
>> On Wed, Sep 21, 2022, Vitaly Kuznetsov wrote:
>> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> > index f62d5799fcd7..86504a8bfd9a 100644
>> > --- a/arch/x86/kvm/x86.c
>> > +++ b/arch/x86/kvm/x86.c
>> > @@ -3418,11 +3418,17 @@ static inline void kvm_vcpu_flush_tlb_current(struct kvm_vcpu *vcpu)
>> >   */
>> >  void kvm_service_local_tlb_flush_requests(struct kvm_vcpu *vcpu)
>> >  {
>> > -	if (kvm_check_request(KVM_REQ_TLB_FLUSH_CURRENT, vcpu))
>> > +	if (kvm_check_request(KVM_REQ_TLB_FLUSH_CURRENT, vcpu)) {
>> >  		kvm_vcpu_flush_tlb_current(vcpu);
>> > +		kvm_clear_request(KVM_REQ_HV_TLB_FLUSH, vcpu);
>> 
>> This isn't correct, flush_tlb_current() flushes "host" TLB entries, i.e. guest-physical
>> mappings in Intel terminology, where flush_tlb_guest() and (IIUC) Hyper-V's paravirt
>> TLB flush both flesh "guest" TLB entries, i.e. linear and combined mappings.
>> 
>> Amusing side topic, apparently I like arm's stage-2 terminology better than "TDP",
>> because I actually typed out "stage-2" first.
>> 
>> > +	}
>> >  
>> > -	if (kvm_check_request(KVM_REQ_TLB_FLUSH_GUEST, vcpu))
>> > +	if (kvm_check_request(KVM_REQ_TLB_FLUSH_GUEST, vcpu)) {
>> > +		kvm_vcpu_flush_tlb_guest(vcpu);
>> > +		kvm_clear_request(KVM_REQ_HV_TLB_FLUSH, vcpu);
>
> Looking at future patches where KVM needs to reset the FIFO when doing a "guest"
> TLB flush, i.e. needs to do more than just clearing the request, what about putting
> this in kvm_vcpu_flush_tlb_guest() right away?

Will do.

>
> Ah, and there's already a second caller to kvm_vcpu_flush_tlb_guest().  I doubt
> KVM's paravirt TLB flush will ever collide with Hyper-V's paravirt TLB flush,
> but logically a "guest" flush that is initiated through KVM's paravirt interface
> should also clear Hyper-V's queue/request.

I ignored this as a case which is not worth optimizing for,
i.e. over-flushing is always correct.

>
> And for consistency, slot this in before this patch:
>

Will do, thanks!

> From: Sean Christopherson <seanjc@...gle.com>
> Date: Wed, 21 Sep 2022 09:35:34 -0700
> Subject: [PATCH] KVM: x86: Move clearing of TLB_FLUSH_CURRENT to
>  kvm_vcpu_flush_tlb_all()
>
> Clear KVM_REQ_TLB_FLUSH_CURRENT in kvm_vcpu_flush_tlb_all() instead of in
> its sole caller that processes KVM_REQ_TLB_FLUSH.  Regardless of why/when
> kvm_vcpu_flush_tlb_all() is called, flushing "all" TLB entries also
> flushes "current" TLB entries.
>
> Ideally, there will never be another caller of kvm_vcpu_flush_tlb_all(),
> and moving the handling "requires" extra work to document the ordering
> requirement, but future Hyper-V paravirt TLB flushing support will add
> similar logic for flush "guest" (Hyper-V can flush a subset of "guest"
> entries).  And in the Hyper-V case, KVM needs to do more than just clear
> the request, the queue of GPAs to flush also needs to purged, and doing
> all only in the request path is undesirable as kvm_vcpu_flush_tlb_guest()
> does have multiple callers (though it's unlikely KVM's paravirt TLB flush
> will coincide with Hyper-V's paravirt TLB flush).
>
> Move the logic even though it adds extra "work" so that KVM will be
> consistent with how flush requests are processed when the Hyper-V support
> lands.
>
> No functional change intended.
>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  arch/x86/kvm/x86.c | 15 ++++++++++-----
>  1 file changed, 10 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index f62d5799fcd7..3ea2e51a8cb5 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -3383,6 +3383,9 @@ static void kvm_vcpu_flush_tlb_all(struct kvm_vcpu *vcpu)
>  {
>  	++vcpu->stat.tlb_flush;
>  	static_call(kvm_x86_flush_tlb_all)(vcpu);
> +
> +	/* Flushing all ASIDs flushes the current ASID... */
> +	kvm_clear_request(KVM_REQ_TLB_FLUSH_CURRENT, vcpu);
>  }
>  
>  static void kvm_vcpu_flush_tlb_guest(struct kvm_vcpu *vcpu)
> @@ -10462,12 +10465,14 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>  			kvm_mmu_sync_roots(vcpu);
>  		if (kvm_check_request(KVM_REQ_LOAD_MMU_PGD, vcpu))
>  			kvm_mmu_load_pgd(vcpu);
> -		if (kvm_check_request(KVM_REQ_TLB_FLUSH, vcpu)) {
> +
> +		/*
> +		 * Note, the order matters here, as flushing "all" TLB entries
> +		 * also flushes the "current" TLB entries, i.e. servicing the
> +		 * flush "all" will clear any request to flush "current".
> +		 */
> +		if (kvm_check_request(KVM_REQ_TLB_FLUSH, vcpu))
>  			kvm_vcpu_flush_tlb_all(vcpu);
> -
> -			/* Flushing all ASIDs flushes the current ASID... */
> -			kvm_clear_request(KVM_REQ_TLB_FLUSH_CURRENT, vcpu);
> -		}
>  		kvm_service_local_tlb_flush_requests(vcpu);
>  
>  		if (kvm_check_request(KVM_REQ_REPORT_TPR_ACCESS, vcpu)) {
>
> base-commit: ed102fe0b59586397b362a849bd7fb32582b77d8

-- 
Vitaly