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From: Paolo Bonzini <pbonzini@redhat.com>
To: Andrea Arcangeli <aarcange@redhat.com>
Cc: Eduardo Habkost <ehabkost@redhat.com>,
	Marcel Apfelbaum <marcel@redhat.com>,
	"Michael S. Tsirkin" <mst@redhat.com>,
	"Dr. David Alan Gilbert" <dgilbert@redhat.com>,
	qemu-devel@nongnu.org
Subject: Re: [Qemu-devel] Default for phys-addr-bits? (was Re: [PATCH 4/5] x86: Allow physical address bits to be set)
Date: Wed, 22 Jun 2016 16:48:50 +0200	[thread overview]
Message-ID: <21a5ea2e-e83b-a874-8992-9e5bde710840@redhat.com> (raw)
In-Reply-To: <20160622144410.GL30202@redhat.com>



On 22/06/2016 16:44, Andrea Arcangeli wrote:
> On Wed, Jun 22, 2016 at 04:33:18PM +0200, Paolo Bonzini wrote:
>>
>>
>> On 22/06/2016 16:24, Andrea Arcangeli wrote:
>>> Linux could not possibly crash instead if host phys bits > guest phys
>>> bits because it will never depend on GPF triggering if the must be
>>> zero bits of the guest pagetables are set. Linux won't ever try to set
>>> those bits and I'd be shocked if any other OS does.
>>
>> Well, KVM does.  It sets _all_ bits up to 51, not just one, but still we
>> have a counterexample.
> 
> How can that crash? KVM doesn't use the host phys bits (or level1 host
> phys bits), the bit to set is hardcoded up to 51 and assumed no host
> would possibly fail at that.

KVM encodes other information in the sPTE when it sets the reserved bit
(a generation count).  Instead of using all bits up to 51, KVM could
well use bit MAXPHYADDR+1 as a marker and add bits MAXPHYADDR+2...51 to
the generation count.

You cannot really rely on what the guest does, no matter how crazy it seems.

Paolo

  reply	other threads:[~2016-06-22 14:49 UTC|newest]

Thread overview: 73+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-06-16 17:12 [Qemu-devel] [PATCH 0/5] x86: Physical address limit patches Dr. David Alan Gilbert (git)
2016-06-16 17:12 ` [Qemu-devel] [PATCH 1/5] BIT_RANGE convenience macro Dr. David Alan Gilbert (git)
2016-06-16 17:23   ` Paolo Bonzini
2016-06-16 17:24     ` Dr. David Alan Gilbert
2016-06-16 18:01   ` Peter Maydell
2016-06-16 18:05     ` Paolo Bonzini
2016-06-20 14:11     ` Dr. David Alan Gilbert
2016-06-20 14:17       ` Peter Maydell
2016-06-16 17:12 ` [Qemu-devel] [PATCH 2/5] x86: Mask mtrr mask based on CPU physical address limits Dr. David Alan Gilbert (git)
2016-06-16 19:59   ` Eduardo Habkost
2016-06-17  8:23     ` Dr. David Alan Gilbert
2016-06-17 12:13     ` Paolo Bonzini
2016-06-16 17:12 ` [Qemu-devel] [PATCH 3/5] x86: fill high bits of mtrr mask Dr. David Alan Gilbert (git)
2016-06-16 20:14   ` Eduardo Habkost
2016-06-17  7:47     ` Paolo Bonzini
2016-06-17 12:46       ` Eduardo Habkost
2016-06-17 13:01         ` Paolo Bonzini
2016-06-17 13:41           ` Eduardo Habkost
2016-06-17 14:25             ` Paolo Bonzini
2016-06-17 15:27               ` Eduardo Habkost
2016-06-17 15:29                 ` Paolo Bonzini
2016-06-17 15:35                   ` Eduardo Habkost
2016-06-17 13:51           ` Dr. David Alan Gilbert
2016-06-17 14:19             ` Paolo Bonzini
2016-06-17  8:53     ` Dr. David Alan Gilbert
2016-06-16 17:12 ` [Qemu-devel] [PATCH 4/5] x86: Allow physical address bits to be set Dr. David Alan Gilbert (git)
2016-06-16 17:26   ` Paolo Bonzini
2016-06-16 18:09     ` Eduardo Habkost
2016-06-16 20:24   ` Eduardo Habkost
2016-06-17  8:15     ` Dr. David Alan Gilbert
2016-06-17  8:43       ` Paolo Bonzini
2016-06-17  9:17         ` Gerd Hoffmann
2016-06-17  9:52           ` Igor Mammedov
2016-06-17 11:20             ` Gerd Hoffmann
2016-06-17 16:20               ` Laszlo Ersek
2016-06-17 16:07             ` Laszlo Ersek
2016-06-19 16:13               ` Marcel Apfelbaum
2016-06-20 10:42                 ` Igor Mammedov
2016-06-20 11:13                   ` Marcel Apfelbaum
2016-06-17  9:37       ` Dr. David Alan Gilbert
2016-06-17  9:54         ` Paolo Bonzini
2016-06-17 13:18       ` Eduardo Habkost
2016-06-17 13:38         ` Paolo Bonzini
2016-06-17 15:19           ` Eduardo Habkost
2016-06-17 15:28             ` Paolo Bonzini
2016-06-17 15:49               ` Eduardo Habkost
2016-06-21 19:44                 ` [Qemu-devel] Default for phys-addr-bits? (was Re: [PATCH 4/5] x86: Allow physical address bits to be set) Eduardo Habkost
2016-06-22 12:41                   ` Paolo Bonzini
2016-06-22 14:24                     ` Andrea Arcangeli
2016-06-22 14:33                       ` Paolo Bonzini
2016-06-22 14:44                         ` Andrea Arcangeli
2016-06-22 14:48                           ` Paolo Bonzini [this message]
2016-06-22 15:02                             ` Andrea Arcangeli
2016-06-22 22:44                       ` Michael S. Tsirkin
2016-06-22 23:23                         ` Andrea Arcangeli
2016-06-22 23:45                           ` Michael S. Tsirkin
2016-06-23  8:40                             ` Gerd Hoffmann
2016-06-23 16:38                               ` Michael S. Tsirkin
2016-06-24  5:55                                 ` Gerd Hoffmann
2016-06-24 23:12                                   ` Michael S. Tsirkin
2016-06-29 16:42                               ` Dr. David Alan Gilbert
2016-06-30  6:10                                 ` Gerd Hoffmann
2016-06-30 10:59                                   ` Dr. David Alan Gilbert
2016-06-30 16:14                                     ` Gerd Hoffmann
2016-06-30 17:12                                       ` Dr. David Alan Gilbert
2016-07-01 19:03                                       ` Dr. David Alan Gilbert
2016-06-22 22:40                     ` Michael S. Tsirkin
2016-06-22 23:15                       ` Andrea Arcangeli
2016-06-19  3:36           ` [Qemu-devel] [PATCH 4/5] x86: Allow physical address bits to be set Michael S. Tsirkin
2016-06-20  7:04             ` Paolo Bonzini
2016-06-17 14:24         ` Marcel Apfelbaum
2016-06-16 17:12 ` [Qemu-devel] [PATCH 5/5] x86: Set physical address bits based on host Dr. David Alan Gilbert (git)
2016-06-17  7:25   ` Igor Mammedov

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