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From: Sean Christopherson <seanjc@google.com>
To: Zeng Guang <guang.zeng@intel.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
	kvm@vger.kernel.org, Dave Hansen <dave.hansen@linux.intel.com>,
	Tony Luck <tony.luck@intel.com>,
	Kan Liang <kan.liang@linux.intel.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Kim Phillips <kim.phillips@amd.com>,
	Jarkko Sakkinen <jarkko@kernel.org>,
	Jethro Beekman <jethro@fortanix.com>,
	Kai Huang <kai.huang@intel.com>,
	x86@kernel.org, linux-kernel@vger.kernel.org,
	Robert Hu <robert.hu@intel.com>, Gao Chao <chao.gao@intel.com>
Subject: Re: [PATCH v4 6/6] KVM: VMX: enable IPI virtualization
Date: Fri, 10 Sep 2021 23:55:25 +0000	[thread overview]
Message-ID: <YTvwbUhofR3Fv7bV@google.com> (raw)
In-Reply-To: <YTvttCcfqF7D/CXt@google.com>

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

  reply	other threads:[~2021-09-10 23:55 UTC|newest]

Thread overview: 21+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-09  3:29 [PATCH v4 0/6] IPI virtualization support for VM Zeng Guang
2021-08-09  3:29 ` [PATCH v4 1/6] x86/feat_ctl: Add new VMX feature, Tertiary VM-Execution control Zeng Guang
2021-09-10 21:25   ` Sean Christopherson
2021-09-17 16:10     ` Zeng Guang
2021-08-09  3:29 ` [PATCH v4 2/6] KVM: VMX: Extend BUILD_CONTROLS_SHADOW macro to support 64-bit variation Zeng Guang
2021-09-10 21:28   ` Sean Christopherson
2021-09-17 16:13     ` Zeng Guang
2021-08-09  3:29 ` [PATCH v4 3/6] KVM: VMX: Detect Tertiary VM-Execution control when setup VMCS config Zeng Guang
2021-09-10 21:35   ` Sean Christopherson
2021-09-17 16:15     ` Zeng Guang
2021-08-09  3:29 ` [PATCH v4 4/6] KVM: VMX: dump_vmcs() reports tertiary_exec_control field as well Zeng Guang
2021-08-09  3:29 ` [PATCH v4 5/6] KVM: x86: Support interrupt dispatch in x2APIC mode with APIC-write VM exit Zeng Guang
2021-09-10 22:28   ` Sean Christopherson
2021-09-17 16:00     ` Zeng Guang
2021-08-09  3:29 ` [PATCH v4 6/6] KVM: VMX: enable IPI virtualization Zeng Guang
2021-09-10 23:43   ` Sean Christopherson
2021-09-10 23:55     ` Sean Christopherson [this message]
2021-09-17 16:10       ` Zeng Guang
2021-10-14 18:56         ` Sean Christopherson
2021-09-17 16:00     ` Zeng Guang
2021-08-20 13:19 ` [PATCH v4 0/6] IPI virtualization support for VM Zeng Guang

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