From: "Michael S. Tsirkin" <mst@redhat.com>
To: Igor Mammedov <imammedo@redhat.com>
Cc: Xiao Guangrong <guangrong.xiao@linux.intel.com>,
ehabkost@redhat.com, ghammer@redhat.com, qemu-devel@nongnu.org,
lcapitulino@redhat.com, lersek@redhat.com
Subject: Re: [Qemu-devel] [PATCH v19 3/9] pc: add a Virtual Machine Generation ID device
Date: Wed, 10 Feb 2016 10:51:47 +0200 [thread overview]
Message-ID: <20160210103628-mutt-send-email-mst@redhat.com> (raw)
In-Reply-To: <20160209114608.4f89b528@nial.brq.redhat.com>
On Tue, Feb 09, 2016 at 11:46:08AM +0100, Igor Mammedov wrote:
> > > > > 2. ACPI approach consumes guest usable RAM to allocate buffer
> > > > > and then makes device to DMA data in that RAM.
> > > > > That's a design point I don't agree with.
> > > >
> > > > Blame the broken VM GEN ID spec.
> > > >
> > > > For VM GEN ID, instead of DMA we could make ACPI code read PCI BAR and
> > > > copy data over, this would fix rebalancing but there is a problem with
> > > > this approach: it can not be done atomically (while VM is not yet
> > > > running and accessing RAM). So you can have guest read a partially
> > > > corrupted ID from memory.
> > >
> > > Yep, VM GEN ID spec is broken and we can't do anything about it as
> > > it's absolutely impossible to guaranty atomic update as guest OS
> > > has address of the buffer and can read it any time. Nothing could be
> > > done here.
> >
> > Hmm I thought we can stop VM while we are changing the ID.
> > But of course VM could be accessing it at the same time.
> > So I take this back, ACPI code reading PCI BAR and
> > writing data out to the buffer would be fine
> > from this point of view.
> spec is broken regardless of who reads BAR or RAM as read
> isn't atomic and UUID could be updated in the middle.
> So MS will have to live with that, unless they have a secret
> way to tell guest stop reading from that ADDR, do update and
> signal guest that it can read it.
And really, that the issue that's driving this up to v19. There's no
good way to implement a bad spec. It feels more like a failed
experiment than like a thought through interface. So maybe we should
leave this alone, wait until we see an actual user - this way we can
figure out the implementation constraints better.
--
MST
next prev parent reply other threads:[~2016-02-10 8:51 UTC|newest]
Thread overview: 59+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-01-28 10:54 [Qemu-devel] [PATCH v19 0/9] Virtual Machine Generation ID Igor Mammedov
2016-01-28 10:54 ` [Qemu-devel] [PATCH v19 1/9] acpi: extend ACPI interface to provide access to ACPI registers and SCI irq Igor Mammedov
2016-01-28 10:54 ` [Qemu-devel] [PATCH v19 2/9] docs: vm generation id device's description Igor Mammedov
2016-01-28 10:54 ` [Qemu-devel] [PATCH v19 3/9] pc: add a Virtual Machine Generation ID device Igor Mammedov
2016-01-28 11:13 ` Michael S. Tsirkin
2016-01-28 12:03 ` Igor Mammedov
2016-01-28 12:59 ` Michael S. Tsirkin
2016-01-29 11:13 ` Igor Mammedov
2016-01-31 16:22 ` Michael S. Tsirkin
2016-02-02 9:59 ` Igor Mammedov
2016-02-02 11:16 ` Michael S. Tsirkin
2016-02-09 10:46 ` Igor Mammedov
2016-02-09 12:17 ` Michael S. Tsirkin
2016-02-11 15:16 ` Igor Mammedov
2016-02-11 16:30 ` Michael S. Tsirkin
2016-02-11 17:34 ` Marcel Apfelbaum
2016-02-12 6:15 ` Michael S. Tsirkin
2016-02-15 10:30 ` Igor Mammedov
2016-02-15 11:26 ` Michael S. Tsirkin
2016-02-15 13:56 ` Igor Mammedov
2016-02-16 10:05 ` Marcel Apfelbaum
2016-02-16 12:17 ` Igor Mammedov
2016-02-16 12:36 ` Marcel Apfelbaum
2016-02-16 13:51 ` Igor Mammedov
2016-02-16 14:53 ` Michael S. Tsirkin
2016-02-16 15:10 ` Michael S. Tsirkin
2016-02-10 8:51 ` Michael S. Tsirkin [this message]
2016-02-10 9:28 ` Michael S. Tsirkin
2016-02-10 10:00 ` Laszlo Ersek
2016-01-28 13:48 ` Laszlo Ersek
2016-01-28 10:54 ` [Qemu-devel] [PATCH v19 4/9] tests: add a unit test for the vmgenid device Igor Mammedov
2016-01-28 10:54 ` [Qemu-devel] [PATCH v19 5/9] qmp/hmp: add query-vm-generation-id and 'info vm-generation-id' commands Igor Mammedov
2016-02-09 17:31 ` Eric Blake
2016-01-28 10:54 ` [Qemu-devel] [PATCH v19 6/9] qmp/hmp: add set-vm-generation-id commands Igor Mammedov
2016-02-09 17:33 ` Eric Blake
2016-01-28 10:54 ` [Qemu-devel] [PATCH v19 8/9] pc: put PIIX3 in slot 1 explicitly and cleanup functions assignment Igor Mammedov
2016-01-28 10:54 ` [Qemu-devel] [PATCH v19 9/9] pc/q53: by default put vmgenid device as an function of ISA bridge Igor Mammedov
2016-01-28 10:58 ` [Qemu-devel] [PATCH v19 7/9] machine: add properties to compat_props incrementaly Igor Mammedov
2016-01-28 14:02 ` Eduardo Habkost
2016-01-28 17:00 ` Igor Mammedov
2016-02-03 17:55 ` [Qemu-devel] qdev & hw/core owner? (was Re: [PATCH v19 7/9] machine: add properties to compat_props incrementaly) Eduardo Habkost
2016-02-03 18:46 ` Laszlo Ersek
2016-02-03 19:06 ` Michael S. Tsirkin
2016-02-04 11:31 ` Paolo Bonzini
2016-02-04 11:41 ` Andreas Färber
2016-02-04 11:55 ` Paolo Bonzini
2016-02-04 12:06 ` Michael S. Tsirkin
2016-02-05 7:49 ` Markus Armbruster
2016-02-05 7:51 ` Marcel Apfelbaum
2016-02-11 19:41 ` Eduardo Habkost
2016-02-12 9:17 ` Marcel Apfelbaum
2016-02-12 11:22 ` Andreas Färber
2016-02-12 18:17 ` Eduardo Habkost
2016-02-12 22:30 ` Paolo Bonzini
2016-02-12 18:09 ` Eduardo Habkost
2016-02-05 7:52 ` Markus Armbruster
2016-02-04 12:03 ` Michael S. Tsirkin
2016-02-04 12:12 ` Marcel Apfelbaum
2016-01-29 12:51 ` [Qemu-devel] [PATCH v19 7/9] machine: add properties to compat_props incrementaly Cornelia Huck
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