From: Rusty Russell <rusty@rustcorp.com.au>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: fes@google.com, aarcange@redhat.com, riel@redhat.com,
kvm@vger.kernel.org, yvugenfi@redhat.com,
linux-kernel@vger.kernel.org, mikew@google.com,
yinghan@google.com, Paolo Bonzini <pbonzini@redhat.com>,
virtualization@lists.linux-foundation.org
Subject: Re: [PATCH] virtio-balloon spec: provide a version of the "silent deflate" feature that works
Date: Sat, 08 Sep 2012 14:36:00 +0930 [thread overview]
Message-ID: <87sjatf6iv.fsf@rustcorp.com.au> (raw)
In-Reply-To: <20120907104306.GA17211@redhat.com>
"Michael S. Tsirkin" <mst@redhat.com> writes:
> On Fri, Sep 07, 2012 at 04:09:50PM +0930, Rusty Russell wrote:
>> > So it looks like a bug: we should teach driver to tell host first on leak?
>> > Yan, Vadim, can you comment please?
>> >
>> > Also if true, looks like this bit will be useful to detect a fixed driver on
>> > the hypervisor side - to avoid unmapping such pages? Rusty what do you
>> > think?
>>
>> So, feature is unimplemented in qemu, and broken in drivers. I starting
>> to share Paolo's dislike of it.
>
> What is broken in drivers?
Because supporting the feature is *not* optional for a driver.
If the device said MUST_TELL_HOST, it meant that the driver *had* to
tell the host before it touched the page, otherwise Bad Things might
happen. It was in the original spec precisely to allow devices to
actually *remove* pages.
Noone ever noticed the windows driver didn't support it, because qemu
never requires MUST_TELL_HOST.
So in practice, it's now an optional feature. Since no device used it
anyway, we're better off discarding it than trying to fix it.
If someone wants an *optional* "tell me first" feature later, that's
easy to add, but I don't see why they'd want to.
> Do we really know there are no hypervisors implementing it?
As much as can be known. Qemu doesn't, lkvm doesn't.
> As I said above drivers do have support.
Not the windows drivers. So it's optional, thus removing it will likely
harm noone.
Cheers,
Rusty.
next prev parent reply other threads:[~2012-09-08 5:06 UTC|newest]
Thread overview: 36+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-09-06 7:46 [PATCH] virtio-balloon spec: provide a version of the "silent deflate" feature that works Paolo Bonzini
2012-09-06 8:47 ` Michael S. Tsirkin
2012-09-06 9:24 ` Paolo Bonzini
2012-09-06 9:44 ` Michael S. Tsirkin
2012-09-06 9:57 ` Paolo Bonzini
2012-09-06 10:53 ` Michael S. Tsirkin
2012-09-06 12:13 ` Paolo Bonzini
2012-09-06 12:51 ` Michael S. Tsirkin
2012-09-06 13:12 ` Paolo Bonzini
2012-09-06 23:45 ` Rusty Russell
2012-09-07 5:42 ` Michael S. Tsirkin
2012-09-07 6:39 ` Rusty Russell
2012-09-07 9:27 ` Paolo Bonzini
2012-09-07 10:53 ` Michael S. Tsirkin
2012-09-07 11:20 ` Paolo Bonzini
2012-09-07 12:17 ` Michael S. Tsirkin
2012-09-07 12:22 ` Paolo Bonzini
2012-09-07 12:44 ` Michael S. Tsirkin
2012-09-07 14:04 ` Paolo Bonzini
2012-09-07 14:25 ` Michael S. Tsirkin
2012-09-07 14:44 ` Paolo Bonzini
2012-09-08 22:22 ` Michael S. Tsirkin
2012-09-10 5:50 ` Paolo Bonzini
2012-09-10 6:03 ` Michael S. Tsirkin
2012-09-10 6:38 ` Paolo Bonzini
2012-09-10 6:47 ` Michael S. Tsirkin
2012-09-10 6:55 ` Paolo Bonzini
2012-09-07 10:43 ` Michael S. Tsirkin
2012-09-08 5:06 ` Rusty Russell [this message]
2012-09-08 10:32 ` Paolo Bonzini
2012-09-08 22:37 ` Michael S. Tsirkin
2012-09-10 1:43 ` Rusty Russell
2012-09-10 5:51 ` Paolo Bonzini
2012-09-10 5:51 ` Michael S. Tsirkin
2012-09-12 6:24 ` Rusty Russell
2012-09-06 23:41 ` Michael S. Tsirkin
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