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From: David Woodhouse <dwmw2@infradead.org>
To: Ankur Arora <ankur.a.arora@oracle.com>,
	Joao Martins <joao.m.martins@oracle.com>
Cc: "Boris Ostrovsky" <boris.ostrovsky@oracle.com>,
	"Paolo Bonzini" <pbonzini@redhat.com>,
	"Radim Krčmář" <rkrcmar@redhat.com>,
	"Thomas Gleixner" <tglx@linutronix.de>,
	"Ingo Molnar" <mingo@redhat.com>,
	"Borislav Petkov" <bp@alien8.de>,
	"H. Peter Anvin" <hpa@zytor.com>,
	x86@kernel.org, linux-kernel@vger.kernel.org,
	kvm@vger.kernel.org
Subject: Re: [PATCH RFC 03/39] KVM: x86/xen: register shared_info page
Date: Thu, 03 Dec 2020 10:16:06 +0000	[thread overview]
Message-ID: <8c92b2f3a8e8829ec85d22091b2fe84794f12f78.camel@infradead.org> (raw)
In-Reply-To: <6ea92fe2-4067-d0e0-b716-16d39a7a6065@oracle.com>

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On Wed, 2020-12-02 at 12:32 -0800, Ankur Arora wrote:
> > On IRC, Paolo told me that permanent pinning causes problems for memory
> > hotplug, and pointed me at the trick we do with an MMU notifier and
> > kvm_vcpu_reload_apic_access_page().
> 
> Okay that answers my question. Thanks for clearing that up.
> 
> Not sure of a good place to document this but it would be good to
> have this written down somewhere. Maybe kvm_map_gfn()?

Trying not to get too distracted by polishing this part, so I can
continue with making more things actually work. But I took a quick look
at the reload_apic_access_page() thing.

AFAICT it works because the access is only from *within* the vCPU, in
guest mode.

So all the notifier has to do is kick all CPUs, which happens when it
calls kvm_make_all_cpus_request(). Thus we are guaranteed that all CPUs
are *out* of guest mode by the time...

    ...er... maybe not by the time the notifier returns, because all 
    we've done is *send* the IPI and we don't know the other CPUs have 
    actually stopped running the guest yet? 

    Maybe there's some explanation of why the actual TLB shootdown 
    truly *will* occur before the page goes away, and some ordering 
    rules which mean our reschedule IPI will happen first? Something 
    like that ideally would have been in a comment in in MMU notifier.

Anyway, I'm not going to fret about that because it clearly doesn't
work for the pvclock/shinfo cases anyway; those are accessed from the
kernel *outside* guest mode. I think we can use a similar trick but
just protect the accesses with kvm->srcu.

The code in my tree now uses kvm_map_gfn() at setup time and leaves the
shinfo page mapped. A bit like the KVM pvclock code, except that I
don't pointlessly map and unmap all the time while leaving the page
pinned anyway.

So all we need to do is kvm_release_pfn() right after mapping it,
leaving the map->hva "unsafe".

Then we hook in to the MMU notifier to unmap it when it goes away (in
RCU style; clearing the pointer variable, synchronize_srcu(), and
*then* unmap, possibly in different range_start/range/range_end
callbacks).

The *users* of such mappings will need an RCU read lock, and will need
to be able to fault the GFN back in when they need it.

For now, though, I'm content that converting the Xen shinfo support to
kvm_map_gfn() is the right way to go, and the memory hotplug support is
an incremental addition on top of it.

And I'm even more comforted by the observation that the KVM pvclock
support *already* pins its pages, so I'm not even failing to meet the
existing bar :)


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  reply	other threads:[~2020-12-03 10:17 UTC|newest]

Thread overview: 126+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-02-20 20:15 [PATCH RFC 00/39] x86/KVM: Xen HVM guest support Joao Martins
2019-02-20 20:15 ` [PATCH RFC 01/39] KVM: x86: fix Xen hypercall page msr handling Joao Martins
2019-02-22  1:30   ` Sean Christopherson
2019-02-22 11:47     ` Joao Martins
2019-02-22 12:51     ` Paolo Bonzini
2020-11-30 10:39       ` David Woodhouse
2020-11-30 11:03         ` Paolo Bonzini
2020-11-30 11:27           ` David Woodhouse
2019-02-20 20:15 ` [PATCH RFC 02/39] KVM: x86/xen: intercept xen hypercalls if enabled Joao Martins
2019-02-21 18:29   ` Sean Christopherson
2019-02-21 20:56     ` Joao Martins
2019-02-22  0:30       ` Sean Christopherson
2019-02-22 12:50         ` Paolo Bonzini
2020-12-01  9:48   ` David Woodhouse
2020-12-01 11:19     ` David Woodhouse
2020-12-02 11:17       ` Joao Martins
2020-12-02 12:12         ` David Woodhouse
2020-12-02  5:19     ` Ankur Arora
2020-12-02  8:03       ` David Woodhouse
2020-12-02 18:20         ` Ankur Arora
2019-02-20 20:15 ` [PATCH RFC 03/39] KVM: x86/xen: register shared_info page Joao Martins
2020-12-01 13:07   ` David Woodhouse
2020-12-02  0:40     ` Ankur Arora
2020-12-02  1:26       ` David Woodhouse
2020-12-02  5:17         ` Ankur Arora
2020-12-02 10:50           ` Joao Martins
2020-12-02 10:44       ` Joao Martins
2020-12-02 12:20         ` David Woodhouse
2020-12-02 20:32           ` Ankur Arora
2020-12-03 10:16             ` David Woodhouse [this message]
2020-12-04 17:30               ` Sean Christopherson
2020-12-02 20:33         ` Ankur Arora
2020-12-12 12:07       ` David Woodhouse
2019-02-20 20:15 ` [PATCH RFC 04/39] KVM: x86/xen: setup pvclock updates Joao Martins
2019-02-20 20:15 ` [PATCH RFC 05/39] KVM: x86/xen: update wallclock region Joao Martins
2019-02-20 20:15 ` [PATCH RFC 06/39] KVM: x86/xen: register vcpu info Joao Martins
2019-02-20 20:15 ` [PATCH RFC 07/39] KVM: x86/xen: register vcpu time info region Joao Martins
2019-02-20 20:15 ` [PATCH RFC 08/39] KVM: x86/xen: register steal clock Joao Martins
2019-02-20 20:15 ` [PATCH RFC 09/39] KVM: x86: declare Xen HVM guest capability Joao Martins
2019-02-20 20:15 ` [PATCH RFC 10/39] KVM: x86/xen: support upcall vector Joao Martins
2020-12-02 11:17   ` David Woodhouse
2020-12-02 13:12     ` Joao Martins
2020-12-02 16:47       ` David Woodhouse
2020-12-02 18:34         ` Joao Martins
2020-12-02 19:02           ` David Woodhouse
2020-12-02 20:12             ` Joao Martins
2020-12-02 20:37               ` David Woodhouse
2020-12-03  1:08             ` Ankur Arora
2020-12-08 16:08             ` David Woodhouse
2020-12-09  6:35               ` Ankur Arora
2020-12-09 10:27                 ` David Woodhouse
2020-12-09 10:51                   ` Joao Martins
2020-12-09 11:39                     ` David Woodhouse
2020-12-09 13:26                       ` Joao Martins
2020-12-09 15:41                         ` David Woodhouse
2020-12-09 16:12                           ` Joao Martins
2021-01-01 14:33           ` David Woodhouse
2021-01-05 12:11             ` Joao Martins
2021-01-05 13:23               ` David Woodhouse
2019-02-20 20:15 ` [PATCH RFC 11/39] KVM: x86/xen: evtchn signaling via eventfd Joao Martins
2020-11-30  9:41   ` David Woodhouse
2020-11-30 12:17     ` Joao Martins
2020-11-30 12:55       ` David Woodhouse
2020-11-30 15:08         ` Joao Martins
2020-11-30 16:48           ` David Woodhouse
2020-11-30 17:15             ` Joao Martins
2020-11-30 18:01               ` David Woodhouse
2020-11-30 18:41                 ` Joao Martins
2020-11-30 19:04                   ` David Woodhouse
2020-11-30 19:25                     ` Joao Martins
2021-11-23 13:15           ` David Woodhouse
2019-02-20 20:15 ` [PATCH RFC 12/39] KVM: x86/xen: store virq when assigning evtchn Joao Martins
     [not found]   ` <b750291466f3c89e0a393e48079c087704b217a5.camel@amazon.co.uk>
2022-02-10 12:17     ` Joao Martins
2022-02-10 15:23       ` [EXTERNAL] " David Woodhouse
2019-02-20 20:15 ` [PATCH RFC 13/39] KVM: x86/xen: handle PV timers oneshot mode Joao Martins
2019-02-20 20:15 ` [PATCH RFC 14/39] KVM: x86/xen: handle PV IPI vcpu yield Joao Martins
2019-02-20 20:15 ` [PATCH RFC 15/39] KVM: x86/xen: handle PV spinlocks slowpath Joao Martins
2022-02-08 12:36   ` David Woodhouse
2022-02-10 12:17     ` Joao Martins
2022-02-10 14:11       ` David Woodhouse
2019-02-20 20:15 ` [PATCH RFC 16/39] KVM: x86: declare Xen HVM evtchn offload capability Joao Martins
2019-02-20 20:15 ` [PATCH RFC 17/39] x86/xen: export vcpu_info and shared_info Joao Martins
2019-02-20 20:15 ` [PATCH RFC 18/39] x86/xen: make hypercall_page generic Joao Martins
2019-02-20 20:15 ` [PATCH RFC 19/39] xen/xenbus: xenbus uninit support Joao Martins
2019-02-20 20:15 ` [PATCH RFC 20/39] xen-blkback: module_exit support Joao Martins
2019-02-25 18:57   ` Konrad Rzeszutek Wilk
2019-02-26 11:20     ` Joao Martins
2019-02-20 20:15 ` [PATCH RFC 21/39] KVM: x86/xen: domid allocation Joao Martins
2019-02-20 20:15 ` [PATCH RFC 22/39] KVM: x86/xen: grant table init Joao Martins
2019-02-20 20:15 ` [PATCH RFC 23/39] KVM: x86/xen: grant table grow support Joao Martins
2019-02-20 20:15 ` [PATCH RFC 24/39] KVM: x86/xen: backend hypercall support Joao Martins
2019-02-20 20:15 ` [PATCH RFC 25/39] KVM: x86/xen: grant map support Joao Martins
2019-02-20 20:15 ` [PATCH RFC 26/39] KVM: x86/xen: grant unmap support Joao Martins
2019-02-20 20:15 ` [PATCH RFC 27/39] KVM: x86/xen: grant copy support Joao Martins
2019-02-20 20:15 ` [PATCH RFC 28/39] KVM: x86/xen: interdomain evtchn support Joao Martins
2019-02-20 20:15 ` [PATCH RFC 29/39] KVM: x86/xen: evtchn unmask support Joao Martins
2019-02-20 20:16 ` [PATCH RFC 30/39] KVM: x86/xen: add additional evtchn ops Joao Martins
2019-02-20 20:16 ` [PATCH RFC 31/39] xen-shim: introduce shim domain driver Joao Martins
2019-02-20 20:16 ` [PATCH RFC 32/39] xen/balloon: xen_shim_domain() support Joao Martins
2019-02-20 20:16 ` [PATCH RFC 33/39] xen/grant-table: " Joao Martins
2019-02-20 20:16 ` [PATCH RFC 34/39] xen/gntdev: " Joao Martins
2019-02-20 20:16 ` [PATCH RFC 35/39] xen/xenbus: " Joao Martins
2019-02-20 20:16 ` [PATCH RFC 36/39] drivers/xen: " Joao Martins
2019-02-20 20:16 ` [PATCH RFC 37/39] xen-netback: " Joao Martins
2019-02-20 20:16 ` [PATCH RFC 38/39] xen-blkback: " Joao Martins
2019-02-20 20:16 ` [PATCH RFC 39/39] KVM: x86: declare Xen HVM Dom0 capability Joao Martins
2019-02-20 21:09 ` [PATCH RFC 00/39] x86/KVM: Xen HVM guest support Paolo Bonzini
2019-02-21  0:29   ` Ankur Arora
2019-02-21 11:45   ` Joao Martins
2019-02-22 16:59     ` Paolo Bonzini
2019-03-12 17:14       ` Joao Martins
2019-04-08  6:44         ` Juergen Gross
2019-04-08 10:36           ` Joao Martins
2019-04-08 10:42             ` Juergen Gross
2019-04-08 17:31               ` Joao Martins
2019-04-09  0:35                 ` Stefano Stabellini
2019-04-10  5:50                   ` [Xen-devel] " Ankur Arora
2019-04-10 20:45                     ` Stefano Stabellini
2019-04-09  5:04                 ` Juergen Gross
2019-04-10  6:55                   ` Ankur Arora
2019-04-10  7:14                     ` Juergen Gross
2019-02-20 23:39 ` [Xen-devel] " Marek Marczykowski-Górecki
2019-02-21  0:31   ` Ankur Arora
2019-02-21  7:57   ` Juergen Gross
2019-02-21 12:00     ` Joao Martins
2019-02-21 11:55   ` Joao Martins

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