linux-kernel - Re: [RFC 06/33] KVM: x86: hyper-v: Introduce VTL awareness to Hyper-V's PV-IPIs

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Date:   Tue, 05 Dec 2023 17:02:40 +0200
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Nicolas Saenz Julienne <nsaenz@...zon.com>, kvm@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, linux-hyperv@...r.kernel.org,
        pbonzini@...hat.com, seanjc@...gle.com, vkuznets@...hat.com,
        anelkz@...zon.com, graf@...zon.com, dwmw@...zon.co.uk,
        jgowans@...zon.com, corbert@....net, kys@...rosoft.com,
        haiyangz@...rosoft.com, decui@...rosoft.com, x86@...nel.org,
        linux-doc@...r.kernel.org
Subject: Re: [RFC 06/33] KVM: x86: hyper-v: Introduce VTL awareness to
 Hyper-V's PV-IPIs

On Fri, 2023-12-01 at 16:31 +0000, Nicolas Saenz Julienne wrote:
> On Tue Nov 28, 2023 at 7:14 AM UTC, Maxim Levitsky wrote:
> > On Wed, 2023-11-08 at 11:17 +0000, Nicolas Saenz Julienne wrote:
> > > HVCALL_SEND_IPI and HVCALL_SEND_IPI_EX allow targeting specific a
> > > specific VTL. Honour the requests.
> > > 
> > > Signed-off-by: Nicolas Saenz Julienne <nsaenz@...zon.com>
> > > ---
> > >  arch/x86/kvm/hyperv.c             | 24 +++++++++++++++++-------
> > >  arch/x86/kvm/trace.h              | 20 ++++++++++++--------
> > >  include/asm-generic/hyperv-tlfs.h |  6 ++++--
> > >  3 files changed, 33 insertions(+), 17 deletions(-)
> > > 
> > > diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
> > > index d4b1b53ea63d..2cf430f6ddd8 100644
> > > --- a/arch/x86/kvm/hyperv.c
> > > +++ b/arch/x86/kvm/hyperv.c
> > > @@ -2230,7 +2230,7 @@ static u64 kvm_hv_flush_tlb(struct kvm_vcpu *vcpu, struct kvm_hv_hcall *hc)
> > >  }
> > > 
> > >  static void kvm_hv_send_ipi_to_many(struct kvm *kvm, u32 vector,
> > > -                                 u64 *sparse_banks, u64 valid_bank_mask)
> > > +                                 u64 *sparse_banks, u64 valid_bank_mask, int vtl)
> > >  {
> > >       struct kvm_lapic_irq irq = {
> > >               .delivery_mode = APIC_DM_FIXED,
> > > @@ -2245,6 +2245,9 @@ static void kvm_hv_send_ipi_to_many(struct kvm *kvm, u32 vector,
> > >                                           valid_bank_mask, sparse_banks))
> > >                       continue;
> > > 
> > > +             if (kvm_hv_get_active_vtl(vcpu) != vtl)
> > > +                     continue;
> > 
> > Do I understand correctly that this is a temporary limitation?
> > In other words, can a vCPU running in VTL1 send an IPI to a vCPU running VTL0,
> > forcing the target vCPU to do async switch to VTL1?
> > I think that this is possible.
> 
> The diff is missing some context. See this simplified implementation
> (when all_cpus is set in the parent function):
> 
> static void kvm_hv_send_ipi_to_many(struct kvm *kvm, u32 vector, int vtl)
> {
> 	[...]
> 	kvm_for_each_vcpu(i, vcpu, kvm) {
> 		if (kvm_hv_get_active_vtl(vcpu) != vtl)
> 			continue;
> 
> 		kvm_apic_set_irq(vcpu, &irq, NULL);
> 	}
> }
> 
> With the one vCPU per VTL approach, kvm_for_each_vcpu() will iterate
> over *all* vCPUs regardless of their VTL. The IPI is targetted at a
> specific VTL, hence the need to filter.
> 
> VTL1 -> VTL0 IPIs are supported and happen (although they are extremely
> rare).

Makes sense now, thanks!

Best regards,
	Maxim Levitsky

> 
> Nicolas
>