linux-kernel - Re: [PATCH v4 6/6] KVM: VMX: enable IPI virtualization

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Message-ID: <YTvwbUhofR3Fv7bV@google.com>
Date:   Fri, 10 Sep 2021 23:55:25 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Zeng Guang <guang.zeng@...el.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Tony Luck <tony.luck@...el.com>,
        Kan Liang <kan.liang@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Kim Phillips <kim.phillips@....com>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        Jethro Beekman <jethro@...tanix.com>,
        Kai Huang <kai.huang@...el.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, Robert Hu <robert.hu@...el.com>,
        Gao Chao <chao.gao@...el.com>
Subject: Re: [PATCH v4 6/6] KVM: VMX: enable IPI virtualization

On Fri, Sep 10, 2021, Sean Christopherson wrote:
> On Mon, Aug 09, 2021, Zeng Guang wrote:
> > +		if (!pages)
> > +			return -ENOMEM;
> > +
> > +		to_kvm_vmx(kvm)->pid_table = (void *)page_address(pages);
> > +		to_kvm_vmx(kvm)->pid_last_index = KVM_MAX_VCPU_ID;
> 
> I don't see the point of pid_last_index if we're hardcoding it to KVM_MAX_VCPU_ID.
> If I understand the ucode pseudocode, there's no performance hit in the happy
> case, i.e. it only guards against out-of-bounds accesses.
> 
> And I wonder if we want to fail the build if this grows beyond an order-1
> allocation, e.g.
> 
> 		BUILD_BUG_ON(PID_TABLE_ORDER > 1);
> 
> Allocating two pages per VM isn't terrible, but 4+ starts to get painful when
> considering the fact that most VMs aren't going to need more than one page.  For
> now I agree the simplicity of not dynamically growing the table is worth burning
> a page.

Ugh, Paolo has queued a series which bumps KVM_MAX_VCPU_ID to 4096[*].  That makes
this an order-3 allocation, which is quite painful.  One thought would be to let
userspace declare the max vCPU it wants to create, not sure if that would work for
xAPIC though.

[*] https://lkml.kernel.org/r/1111efc8-b32f-bd50-2c0f-4c6f506b544b@redhat.com