[GIT PULL] kvm oops fix

[GIT PULL] kvm oops fix

[Avi Kivity]

Linus,

Please pull from the 'linus' branch of

  git://git.kernel.org/pub/scm/linux/kernel/git/avi/kvm.git

To get a one-liner fixing a host oops running  non-pae guests.

Avi Kivity (1):
      KVM: Fix off-by-one when writing to a nonpae guest pde

---

 drivers/kvm/mmu.c |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
commit 6b8d0f9b180cb93513bb65f705b299370f0357a1
Author: Avi Kivity <avi@qumranet.com>
Date:   Wed Apr 18 11:18:18 2007 +0300

    KVM: Fix off-by-one when writing to a nonpae guest pde
    
    Nonpae guest pdes are shadowed by two pae ptes, so we double the offset
    twice: once to account for the pte size difference, and once because we
    need to shadow pdes for a single guest pde.
    
    But when writing to the upper guest pde we also need to truncate the
    lower bits, otherwise the multiply shifts these bits into the pde index
    and causes an access to the wrong shadow pde.  If we're at the end of the
    page (accessing the very last guest pde) we can even overflow into the
    next host page and oops.
    
    Signed-off-by: Avi Kivity <avi@qumranet.com>

diff --git a/drivers/kvm/mmu.c b/drivers/kvm/mmu.c
index e85b4c7..cab26f3 100644
--- a/drivers/kvm/mmu.c
+++ b/drivers/kvm/mmu.c
@@ -1171,6 +1171,7 @@ void kvm_mmu_pre_write(struct kvm_vcpu *vcpu, gpa_t gpa, int bytes)
 			 * and zap two pdes instead of one.
 			 */
 			if (level == PT32_ROOT_LEVEL) {
+				page_offset &= ~7; /* kill rounding error */
 				page_offset <<= 1;
 				npte = 2;
 			}

Re: [GIT PULL] kvm oops fix

[Linus Torvalds]

On Thu, 19 Apr 2007, Avi Kivity wrote:
> 
> Please pull from the 'linus' branch of
> 
>   git://git.kernel.org/pub/scm/linux/kernel/git/avi/kvm.git

*please* put the branch-name after the git repo, so that I can 
cut-and-paste without noticing only afterwards that the diffstat doesn't 
match what it was supposed to, and I got the wrong commits, and have to 
undo and re-do the pull..

Yeah, I didn't do that mistake this time, and in fact, I seldom do, but 
every time the branch is mentioned somewhere else than with the actual 
repo to pull, I get nervous. 

So I've said this a million times before, but I'll say it one more 
time: if it's not the main branch, the message should be

   Please pull from the 'linus' branch of

     git://git.kernel.org/pub/scm/linux/kernel/git/avi/kvm.git linus

   to get the following changes: ...

so that when I cut-and-paste the URL (you didn't think I'd try to type it 
in by hand, did you?) I get the branch automatically.

		Linus

Re: [GIT PULL] kvm oops fix

[Jeff Garzik]

Linus Torvalds wrote:
> 
> On Thu, 19 Apr 2007, Avi Kivity wrote:
>> Please pull from the 'linus' branch of
>>
>>   git://git.kernel.org/pub/scm/linux/kernel/git/avi/kvm.git
> 
> *please* put the branch-name after the git repo, so that I can 
> cut-and-paste without noticing only afterwards that the diffstat doesn't 
> match what it was supposed to, and I got the wrong commits, and have to 
> undo and re-do the pull..

What is the easiest way to completely undo a pull, reverting the branch 
to the HEAD present before the pull?

	Jeff

Re: [kvm-devel] [GIT PULL] kvm oops fix

[David Brown]

> What is the easiest way to completely undo a pull, reverting the branch
> to the HEAD present before the pull?
>

If the pull doesn't merge successfully then usually doing a `git-reset
--hard` will blow everything away back to normal, but Linus may do
different things.

- David Brown

Re: [kvm-devel] [GIT PULL] kvm oops fix

[Jeff Garzik]

David Brown wrote:
>> What is the easiest way to completely undo a pull, reverting the branch
>> to the HEAD present before the pull?
>>
> 
> If the pull doesn't merge successfully then usually doing a `git-reset
> --hard` will blow everything away back to normal, but Linus may do
> different things.

I'm thinking about a successful pull that one later regrets :)

	Jeff

Re: [kvm-devel] [GIT PULL] kvm oops fix

[J. Bruce Fields]

On Thu, Apr 19, 2007 at 03:34:01PM -0400, Jeff Garzik wrote:
> David Brown wrote:
> >>What is the easiest way to completely undo a pull, reverting the branch
> >>to the HEAD present before the pull?
> >>
> >
> >If the pull doesn't merge successfully then usually doing a `git-reset
> >--hard` will blow everything away back to normal, but Linus may do
> >different things.
> 
> I'm thinking about a successful pull that one later regrets :)

git reset --hard ORIG_HEAD, if the pull is the last thing you did.

Otherwise maybe fire up gitk, look for the point you'd like to revert
to, and cut-n-paste the SHA1 ID to the git reset --hard commandline.

--b.

Re: [GIT PULL] kvm oops fix

[Linus Torvalds]

On Thu, 19 Apr 2007, Jeff Garzik wrote:
> 
> What is the easiest way to completely undo a pull, reverting the branch to the
> HEAD present before the pull?

You can either do

	git reset --hard ORIG_HEAD

(git will set ORIG_HEAD before things like pulls or resets, so you can 
always go back), or, if you have reflogs enabled (and if you set up your 
repository with a modern git version it probably will be enabled by 
default), you can just do

	git reset --hard @{1}

where "@{1}" just means "HEAD ref state one change ago" (the same way you 
can say "@{2.hours.ago}" to mean HEAD state two hours ago).

In either case, double-check that that is indeed the version you want to 
revert to with

	git log ORIG_HEAD
 or
	git log @{1}

first, since obviously if you give "git reset --hard" the wrong version, 
it will reset to the wrong state. Although especially with reflogs, your 
previous state will always be logged, so you can always re-do what you 
undid by (again) doing "git reset --hard @{1}" to get back the previous 
state ;)

ALSO! Make sure that you don't have any dirty state in your working tree 
that you don't want to lose! "git reset --hard" will do what it implies: 
it will reset your tree. Very much including throwing away all your dirty 
state (and that you can't get back by going to a previous commit, since 
it was never committed!)

			Linus

Re: [GIT PULL] kvm oops fix

[Linus Torvalds]

On Thu, 19 Apr 2007, Linus Torvalds wrote:
> 
> You can either do
> 
> 	git reset --hard ORIG_HEAD
> 	git reset --hard @{1}

Btw, on the same kind of subject: the whole "what was my previous HEAD" 
issues are obviously also how you'd generally want to see what those 
new patches were, regardless of whether you want to undo them or not.

So it might be worth repeating for people what I do after any pull that I 
feel I want to give a quick look-over.. A simple

	gitk ORIG_HEAD..
or
	gitk HEAD@{1}..
or
	gitk @{1}..
or
	gitk @{12.hours.ago}..

are all variations of the same theme: show what is new since either "last 
update" or "what I had in my tree 12 hours ago".

Btw, the

	gitk @{12.hours.ago}..

thing is very different from

	gitk --since=12.hours.ago

even if they involve the same date.

The "@{12.hours.ago}" syntax pinpoints a particular *commit*, namely what 
your HEAD was pointing at 12 hours ago. So it's literally about your 
particular repository history (give a branch name if you want to specify 
one: so "for-linus@{2.hours.ago}" specifies the *commit* that was the head 
of the "for-linus" branch in your repository 2 hours ago).

In contrast, the "--since=12.hours.ago" means something totally different: 
it measn that you want to ignore all commits that are older than 12 hours, 
regardless of whether they were actually in your tree at that point or 
not. Which is often a very different issue indeed.

So another reasonably common things you can do:

	git fetch linus
	gitk linus@{1}..linus

this assumes that you've set up a separate tracking branch "linus", and 
that you've taught it to fetch my current tree into it. So in the above 
sequence, the "git fetch linus" will fetch everything new from my tree 
into your "linus" tracking branch, and the "gitk" will then show all the 
new commits on that branch that you got.

NOTE! The above is very much designed to work whether you are on that 
branch or not, and in fact, the normal reason to do something like the 
above is explicitly that you want to see what is going on in somebody 
elses tree without actually necessarily merging it into your own branch 
(perhaps in order to decide whether you _want_ to merge it or not).

And that "linus@{1}" really just means "what is the previous commit I had 
on my 'linus' branch". You can obviously dig deeper down, and "linus@{10}" 
is something less commonly used, but basically means "what was on that 
branch ten revision updates ago".

Note that this is *very*different* from "linus~10", which means "what is 
the tenth _parent_ of the "linus" branch. They *can* be the same thing (if 
each operation adds exactly one commit), but if you do things like "git 
fetch", then the "linus" branch ten operations ago may be hundreds of 
commits ago, because some of those ten operations may have added lots of 
commits thanks to synching up with some other tree!

And as already noted, the "branch@{xyzzy}" format also allows "xyzzy" to 
be a date, not just a numeral. In fact, that was the original revlog 
tracking behaviour, and the numeric thing, while simpler, is actually a 
newer feature (as is the "don't specify a branch name at all", which just 
means "current branch")

So

	gitk @{24.hours.ago}..

is a nice way to see what has happend in *your* repository, on the current 
branch, in the last 24 hours.

(NOTE: You can also say "HEAD@{2.hours.ago}" and that actually doesn't use 
the current branch at all, it actually says what HEAD was 2 hours ago: you 
may have been on some totally _different_ branch back then, and if you 
wonder what the heck of a branch you are running and you look at the time 
of the binary, but you don't remember what branch you had checked out when 
you built it, that may be what you want. Of course, you may also want a 
better attention span ;)

Some of this is pretty recent, and generally, if some of this doesn't work 
for you, it means that you are using some ancient version of git. If it's 
not git-1.5.x, upgrade. It's worth it.

		Linus

Re: [kvm-devel] [GIT PULL] kvm oops fix

[David Brown]

On 4/19/07, Avi Kivity <avi@qumranet.com> wrote:
> Linus,
>
> Please pull from the 'linus' branch of
>
>   git://git.kernel.org/pub/scm/linux/kernel/git/avi/kvm.git
>
> To get a one-liner fixing a host oops running  non-pae guests.
>
> Avi Kivity (1):
>       KVM: Fix off-by-one when writing to a nonpae guest pde

Ooo I thought of something else.
Should this be applied to the current 2.6.20.7 for the next 2.6.20.8 release?

- David Brown

Re: [kvm-devel] [GIT PULL] kvm oops fix

[Avi Kivity]

David Brown wrote:
> On 4/19/07, Avi Kivity <avi@qumranet.com> wrote:
>> Linus,
>>
>> Please pull from the 'linus' branch of
>>
>>   git://git.kernel.org/pub/scm/linux/kernel/git/avi/kvm.git
>>
>> To get a one-liner fixing a host oops running  non-pae guests.
>>
>> Avi Kivity (1):
>>       KVM: Fix off-by-one when writing to a nonpae guest pde
>
> Ooo I thought of something else.
> Should this be applied to the current 2.6.20.7 for the next 2.6.20.8
> release?
>

Yes.  I'll prepare a patch.


-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.