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

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" <kvm@vger.kernel.org>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	"Luck, Tony" <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>,
	"Huang, Kai" <kai.huang@intel.com>,
	"x86@kernel.org" <x86@kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"Hu, Robert" <robert.hu@intel.com>,
	"Gao, Chao" <chao.gao@intel.com>
Subject: Re: [PATCH v4 6/6] KVM: VMX: enable IPI virtualization
Date: Thu, 14 Oct 2021 18:56:35 +0000	[thread overview]
Message-ID: <YWh9YyulGLBHBvEt@google.com> (raw)
In-Reply-To: <c13b1ad7-f250-1f7f-4f6c-886b81b06517@intel.com>

On Sat, Sep 18, 2021, Zeng Guang wrote:
> On 9/11/2021 7:55 AM, Sean Christopherson wrote:
> > 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
> Thus we keep current design as no change.

Not necessarily.  I was pointing out that the current design is already problematic
from a memory allocation perspective.  Burning a few pages per vCPU isn't the end
of the world, but 32kb of _contiguous_ memory is rough, especially when 28kb is
unlikely to be used in many cases.