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From: Sean Christopherson <sean.j.christopherson@intel.com>
To: Andy Lutomirski <luto@amacapital.net>
Cc: Andy Lutomirski <luto@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Alexandre Chartre <alexandre.chartre@oracle.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Radim Krcmar <rkrcmar@redhat.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	kvm list <kvm@vger.kernel.org>, X86 ML <x86@kernel.org>,
	Linux-MM <linux-mm@kvack.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
	jan.setjeeilers@oracle.com, Liran Alon <liran.alon@oracle.com>,
	Jonathan Adams <jwadams@google.com>
Subject: Re: [RFC KVM 18/27] kvm/isolation: function to copy page table entries for percpu buffer
Date: Tue, 14 May 2019 15:38:23 -0700	[thread overview]
Message-ID: <20190514223823.GE1977@linux.intel.com> (raw)
In-Reply-To: <A1EB80C0-2D88-4DC0-A898-3BED50A4F5A8@amacapital.net>

On Tue, May 14, 2019 at 02:55:18PM -0700, Andy Lutomirski wrote:
> 
> > On May 14, 2019, at 2:06 PM, Sean Christopherson <sean.j.christopherson@intel.com> wrote:
> > 
> >> On Tue, May 14, 2019 at 01:33:21PM -0700, Andy Lutomirski wrote:
> >> I suspect that the context switch is a bit of a red herring.  A
> >> PCID-don't-flush CR3 write is IIRC under 300 cycles.  Sure, it's slow,
> >> but it's probably minor compared to the full cost of the vm exit.  The
> >> pain point is kicking the sibling thread.
> > 
> > Speaking of PCIDs, a separate mm for KVM would mean consuming another
> > ASID, which isn't good.
> 
> I’m not sure we care. We have many logical address spaces (two per mm plus a
> few more).  We have 4096 PCIDs, but we only use ten or so.  And we have some
> undocumented number of *physical* ASIDs with some undocumented mechanism by
> which PCID maps to a physical ASID.

Yeah, I was referring to physical ASIDs.

> I don’t suppose you know how many physical ASIDs we have?

Limited number of physical ASIDs.  I'll leave it at that so as not to
disclose something I shouldn't.

> And how it interacts with the VPID stuff?

VPID and PCID get factored into the final ASID, i.e. changing either one
results in a new ASID.  The SDM's oblique way of saying that:

  VPIDs and PCIDs (see Section 4.10.1) can be used concurrently. When this
  is done, the processor associates cached information with both a VPID and
  a PCID. Such information is used only if the current VPID and PCID both
  match those associated with the cached information.

E.g. enabling PTI in both the host and guest consumes four ASIDs just to
run a single task in the guest:

  - VPID=0, PCID=kernel
  - VPID=0, PCID=user
  - VPID=1, PCID=kernel
  - VPID=1, PCID=user

The impact of consuming another ASID for KVM would likely depend on both
the guest and host configurations/worloads, e.g. if the guest is using a
lot of PCIDs then it's probably a moot point.  It's something to keep in
mind though if we go down this path.

  reply	other threads:[~2019-05-14 22:38 UTC|newest]

Thread overview: 86+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-05-13 14:38 [RFC KVM 00/27] KVM Address Space Isolation Alexandre Chartre
2019-05-13 14:38 ` [RFC KVM 01/27] kernel: Export memory-management symbols required for KVM address space isolation Alexandre Chartre
2019-05-13 15:15   ` Peter Zijlstra
2019-05-13 15:17     ` Liran Alon
2019-05-13 14:38 ` [RFC KVM 02/27] KVM: x86: Introduce address_space_isolation module parameter Alexandre Chartre
2019-05-13 15:46   ` Andy Lutomirski
2019-05-13 15:55     ` Alexandre Chartre
2019-05-13 14:38 ` [RFC KVM 03/27] KVM: x86: Introduce KVM separate virtual address space Alexandre Chartre
2019-05-13 15:45   ` Andy Lutomirski
2019-05-13 16:04     ` Alexandre Chartre
2019-05-13 14:38 ` [RFC KVM 04/27] KVM: x86: Switch to KVM address space on entry to guest Alexandre Chartre
2019-05-13 14:38 ` [RFC KVM 05/27] KVM: x86: Add handler to exit kvm isolation Alexandre Chartre
2019-05-13 15:49   ` Andy Lutomirski
2019-05-13 16:10     ` Alexandre Chartre
2019-05-13 14:38 ` [RFC KVM 06/27] KVM: x86: Exit KVM isolation on IRQ entry Alexandre Chartre
2019-05-13 15:51   ` Andy Lutomirski
2019-05-13 16:28     ` Alexandre Chartre
2019-05-13 18:13       ` Andy Lutomirski
2019-05-14  7:07         ` Peter Zijlstra
2019-05-14  7:58           ` Alexandre Chartre
2019-05-13 14:38 ` [RFC KVM 07/27] KVM: x86: Switch to host address space when may access sensitive data Alexandre Chartre
2019-05-13 14:38 ` [RFC KVM 08/27] KVM: x86: Optimize branches which checks if address space isolation enabled Alexandre Chartre
2019-05-13 14:38 ` [RFC KVM 09/27] kvm/isolation: function to track buffers allocated for the KVM page table Alexandre Chartre
2019-05-13 14:38 ` [RFC KVM 10/27] kvm/isolation: add KVM page table entry free functions Alexandre Chartre
2019-05-13 14:38 ` [RFC KVM 11/27] kvm/isolation: add KVM page table entry offset functions Alexandre Chartre
2019-05-13 14:38 ` [RFC KVM 12/27] kvm/isolation: add KVM page table entry allocation functions Alexandre Chartre
2019-05-13 14:38 ` [RFC KVM 13/27] kvm/isolation: add KVM page table entry set functions Alexandre Chartre
2019-05-13 14:38 ` [RFC KVM 14/27] kvm/isolation: functions to copy page table entries for a VA range Alexandre Chartre
2019-05-13 14:38 ` [RFC KVM 15/27] kvm/isolation: keep track of VA range mapped in KVM address space Alexandre Chartre
2019-05-13 14:38 ` [RFC KVM 16/27] kvm/isolation: functions to clear page table entries for a VA range Alexandre Chartre
2019-05-13 14:38 ` [RFC KVM 17/27] kvm/isolation: improve mapping copy when mapping is already present Alexandre Chartre
2019-05-13 14:38 ` [RFC KVM 18/27] kvm/isolation: function to copy page table entries for percpu buffer Alexandre Chartre
2019-05-13 18:18   ` Andy Lutomirski
2019-05-14  7:09     ` Peter Zijlstra
2019-05-14  8:25       ` Alexandre Chartre
2019-05-14  8:34         ` Andy Lutomirski
2019-05-14  9:41           ` Alexandre Chartre
2019-05-14 15:23             ` Andy Lutomirski
2019-05-14 16:24               ` Alexandre Chartre
2019-05-14 17:05                 ` Peter Zijlstra
2019-05-14 18:09                   ` Sean Christopherson
2019-05-14 20:33                     ` Andy Lutomirski
2019-05-14 21:06                       ` Sean Christopherson
2019-05-14 21:55                         ` Andy Lutomirski
2019-05-14 22:38                           ` Sean Christopherson [this message]
2019-05-18  0:05                             ` Jonathan Adams
2019-05-14 20:27                   ` Andy Lutomirski
2019-05-13 14:38 ` [RFC KVM 19/27] kvm/isolation: initialize the KVM page table with core mappings Alexandre Chartre
2019-05-13 15:50   ` Dave Hansen
2019-05-13 16:00     ` Andy Lutomirski
2019-05-13 17:00       ` Alexandre Chartre
2019-05-13 16:46     ` Sean Christopherson
2019-05-13 16:47     ` Alexandre Chartre
2019-05-14 10:26       ` Alexandre Chartre
2019-05-13 14:38 ` [RFC KVM 20/27] kvm/isolation: initialize the KVM page table with vmx specific data Alexandre Chartre
2019-05-13 14:38 ` [RFC KVM 21/27] kvm/isolation: initialize the KVM page table with vmx VM data Alexandre Chartre
2019-05-13 14:38 ` [RFC KVM 22/27] kvm/isolation: initialize the KVM page table with vmx cpu data Alexandre Chartre
2019-05-13 14:38 ` [RFC KVM 23/27] kvm/isolation: initialize the KVM page table with the vcpu tasks Alexandre Chartre
2019-05-13 14:38 ` [RFC KVM 24/27] kvm/isolation: KVM page fault handler Alexandre Chartre
2019-05-13 15:15   ` Peter Zijlstra
2019-05-13 21:25     ` Liran Alon
2019-05-14  2:02       ` Andy Lutomirski
2019-05-14  7:21         ` Peter Zijlstra
2019-05-14 15:36           ` Alexandre Chartre
2019-05-14 15:43             ` Andy Lutomirski
2019-05-13 16:02   ` Andy Lutomirski
2019-05-13 16:21     ` Alexandre Chartre
2019-05-13 14:38 ` [RFC KVM 25/27] kvm/isolation: implement actual KVM isolation enter/exit Alexandre Chartre
2019-05-13 15:16   ` Peter Zijlstra
2019-05-13 16:01   ` Andy Lutomirski
2019-05-13 14:38 ` [RFC KVM 26/27] kvm/isolation: initialize the KVM page table with KVM memslots Alexandre Chartre
2019-05-13 14:38 ` [RFC KVM 27/27] kvm/isolation: initialize the KVM page table with KVM buses Alexandre Chartre
2019-05-13 16:42 ` [RFC KVM 00/27] KVM Address Space Isolation Liran Alon
2019-05-13 18:17 ` Andy Lutomirski
2019-05-13 21:08   ` Liran Alon
2019-05-14  2:07     ` Andy Lutomirski
2019-05-14  7:37       ` Peter Zijlstra
2019-05-14 21:32         ` Jan Setje-Eilers
2019-05-14  8:05       ` Liran Alon
2019-05-14  7:29     ` Peter Zijlstra
2019-05-14  7:57       ` Liran Alon
2019-05-14  8:33     ` Alexandre Chartre
2019-05-13 19:31 ` Nakajima, Jun
2019-05-13 21:16   ` Liran Alon
     [not found]     ` <D07C8F51-F2DF-4C8B-AB3B-0DFABD5F4C33@intel.com>
2019-05-13 21:53       ` Liran Alon
2019-05-15 12:52 ` Alexandre Chartre

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