From: "Michael S. Tsirkin" <mst@redhat.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Igor Mammedov <imammedo@redhat.com>,
linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
andrey@xdel.ru
Subject: Re: [PATCH 3/5] vhost: support upto 509 memory regions
Date: Thu, 18 Jun 2015 16:47:33 +0200 [thread overview]
Message-ID: <20150618164559-mutt-send-email-mst@redhat.com> (raw)
In-Reply-To: <5582CBA6.5070105@redhat.com>
On Thu, Jun 18, 2015 at 03:46:14PM +0200, Paolo Bonzini wrote:
>
>
> On 18/06/2015 15:19, Michael S. Tsirkin wrote:
> > On Thu, Jun 18, 2015 at 01:50:32PM +0200, Paolo Bonzini wrote:
> >>
> >>
> >> On 18/06/2015 13:41, Michael S. Tsirkin wrote:
> >>> On Thu, Jun 18, 2015 at 01:39:12PM +0200, Igor Mammedov wrote:
> >>>> Lets leave decision upto users instead of making them live with
> >>>> crashing guests.
> >>>
> >>> Come on, let's fix it in userspace.
> >>
> >> It's not trivial to fix it in userspace. Since QEMU uses RCU there
> >> isn't a single memory map to use for a linear gpa->hva map.
> >
> > Could you elaborate?
> >
> > I'm confused by this mention of RCU.
> > You use RCU for accesses to the memory map, correct?
> > So memory map itself is a write side operation, as such all you need to
> > do is take some kind of lock to prevent conflicting with other memory
> > maps, do rcu sync under this lock.
>
> You're right, the problem isn't directly related to RCU. RCU would be
> easy to handle by using synchronize_rcu instead of call_rcu. While I
> identified an RCU-related problem with Igor's patches, it's much more
> entrenched.
>
> RAM can be used by asynchronous operations while the VM runs, between
> address_space_map and address_space_unmap. It is possible and common to
> have a quiescent state between the map and unmap, and a memory map
> change can happen in the middle of this. Normally this is not a
> problem, because changes to the memory map do not make the hva go away
> (memory regions are reference counted).
Right, so you want mmap(MAP_NORESERVE) when that reference
count becomes 0.
> However, with Igor's patches a memory_region_del_subregion will cause a
> mmap(MAP_NORESERVE), which _does_ have the effect of making the hva go away.
>
> I guess one way to do it would be to alias the same page in two places,
> one for use by vhost and one for use by everything else. However, the
> kernel does not provide the means to do this kind of aliasing for
> anonymous mmaps.
>
> Paolo
Basically pages go away on munmap, so won't simple
lock
munmap
mmap(MAP_NORESERVE)
unlock
do the trick?
--
MST
next prev parent reply other threads:[~2015-06-18 14:47 UTC|newest]
Thread overview: 80+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-06-16 16:33 [PATCH 0/5] vhost: support upto 509 memory regions Igor Mammedov
2015-06-16 16:33 ` [PATCH 1/5] vhost: use binary search instead of linear in find_region() Igor Mammedov
2015-06-16 21:07 ` Michael S. Tsirkin
2015-06-16 21:13 ` Igor Mammedov
2015-06-16 16:33 ` [PATCH 2/5] vhost: extend memory regions allocation to vmalloc Igor Mammedov
2015-06-16 16:33 ` [PATCH 3/5] vhost: support upto 509 memory regions Igor Mammedov
2015-06-16 21:14 ` Michael S. Tsirkin
2015-06-16 22:00 ` Igor Mammedov
2015-06-17 6:34 ` Michael S. Tsirkin
2015-06-17 7:28 ` Igor Mammedov
2015-06-17 7:39 ` Michael S. Tsirkin
2015-06-17 8:54 ` Igor Mammedov
2015-06-17 10:11 ` Michael S. Tsirkin
2015-06-17 10:37 ` Igor Mammedov
2015-06-17 10:46 ` Michael S. Tsirkin
2015-06-17 11:48 ` Igor Mammedov
2015-06-17 11:51 ` Michael S. Tsirkin
2015-06-17 12:23 ` Igor Mammedov
2015-06-17 13:13 ` Michael S. Tsirkin
2015-06-17 13:20 ` Paolo Bonzini
2015-06-17 14:32 ` Michael S. Tsirkin
2015-06-17 15:12 ` Igor Mammedov
2015-06-17 15:38 ` Michael S. Tsirkin
2015-06-17 16:09 ` Igor Mammedov
2015-06-17 16:30 ` Michael S. Tsirkin
2015-06-17 16:31 ` Paolo Bonzini
2015-06-17 16:34 ` Michael S. Tsirkin
2015-06-17 16:38 ` Paolo Bonzini
2015-06-17 16:41 ` Michael S. Tsirkin
2015-06-17 16:47 ` Paolo Bonzini
2015-06-17 17:32 ` Igor Mammedov
2015-06-17 19:11 ` Michael S. Tsirkin
2015-06-17 17:30 ` Igor Mammedov
2015-06-18 9:12 ` Igor Mammedov
2015-06-18 9:50 ` Michael S. Tsirkin
2015-06-18 10:03 ` Paolo Bonzini
2015-06-18 11:39 ` Igor Mammedov
2015-06-18 11:41 ` Michael S. Tsirkin
2015-06-18 11:50 ` Paolo Bonzini
2015-06-18 13:19 ` Michael S. Tsirkin
2015-06-18 13:46 ` Paolo Bonzini
2015-06-18 14:47 ` Michael S. Tsirkin [this message]
2015-06-18 15:54 ` Igor Mammedov
2015-06-18 16:02 ` Paolo Bonzini
2015-06-19 7:56 ` Michael S. Tsirkin
2015-06-19 7:57 ` Paolo Bonzini
2015-06-19 8:05 ` Michael S. Tsirkin
2015-06-19 8:52 ` Paolo Bonzini
2015-06-19 10:14 ` Michael S. Tsirkin
2015-06-19 10:44 ` Paolo Bonzini
2015-06-19 13:34 ` Michael S. Tsirkin
2015-06-19 13:34 ` Michael S. Tsirkin
2015-06-19 15:19 ` Paolo Bonzini
2015-06-19 15:19 ` Paolo Bonzini
2015-06-19 16:20 ` Michael S. Tsirkin
2015-06-19 16:20 ` Michael S. Tsirkin
2015-06-19 16:26 ` Paolo Bonzini
2015-06-19 16:26 ` Paolo Bonzini
2015-06-19 16:33 ` Michael S. Tsirkin
2015-06-19 16:33 ` Michael S. Tsirkin
2015-06-19 16:44 ` Paolo Bonzini
2015-06-19 16:44 ` Paolo Bonzini
2015-06-22 7:10 ` Igor Mammedov
2015-06-22 7:10 ` Igor Mammedov
2015-06-22 9:45 ` Paolo Bonzini
2015-06-22 9:45 ` Paolo Bonzini
2015-06-19 16:45 ` Michael S. Tsirkin
2015-06-19 16:45 ` Michael S. Tsirkin
2015-06-19 16:50 ` Paolo Bonzini
2015-06-19 16:50 ` Paolo Bonzini
2015-06-18 12:02 ` Igor Mammedov
2015-06-17 8:53 ` Paolo Bonzini
2015-06-16 16:33 ` [PATCH 4/5] vhost: add per VQ memory region caching Igor Mammedov
2015-06-16 16:33 ` [PATCH 5/5] vhost: translate_desc: optimization for desc.len < region size Igor Mammedov
2015-06-16 21:11 ` Michael S. Tsirkin
2015-06-16 21:16 ` [PATCH 0/5] vhost: support upto 509 memory regions Michael S. Tsirkin
2015-06-16 22:19 ` Igor Mammedov
2015-06-17 6:31 ` Michael S. Tsirkin
2015-06-17 7:33 ` Igor Mammedov
2015-06-17 7:40 ` Michael S. Tsirkin
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