Grant table version

Grant table version

[Vladimir 'φ-coder/phcoder' Serbinenko]

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Hello, all. I'm working on pvgrub2 and my problem is following:
When you first use grant tables you commit yourself to a particular
grant table version. GRUB has to read from disk and hence use grant
tables. By doing so it commits anything that it loads to the same version.
Would it be possible to have a hypercall to reset grant table version?
Of course, before doing such a call one would need to revoke all grants
or this call would discard all current grants.


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Re: [Xen-devel] Grant table version

[Andrew Cooper]

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On 28/10/13 18:06, Vladimir 'φ-coder/phcoder' Serbinenko wrote:
> Hello, all. I'm working on pvgrub2 and my problem is following:
> When you first use grant tables you commit yourself to a particular
> grant table version. GRUB has to read from disk and hence use grant
> tables. By doing so it commits anything that it loads to the same version.
> Would it be possible to have a hypercall to reset grant table version?
> Of course, before doing such a call one would need to revoke all grants
> or this call would discard all current grants.

You can change back and forth with grant table versions using the
GNTTABOP_set_version hypercall, so long as you have no active grants.

It is not possible to revoke a grant, as the grant ABI guarantees that a
successful grant stays mapped until explicitly unmapped.  Otherwise, a
reset would cause unexpected pagefaults in the mapper domain.

For compatibility reasons it is not strictly required, but it is
certainly expected that a new kernel explicitly chooses a gnttab version
using the hypercall.  There is certainly an area for problems if pvgrub2
uses grant v2, then leaves v2 active and hands off to an older kernel
who expects v1 and doesn't explicitly set the version.

From this point of view, the safe course of action is to use whichever
type of grants you want, then tear down all the front/back pairs, and
reset the version to v1 before handing off.

~Andrew

>
>
>
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@lists.xen.org
> http://lists.xen.org/xen-devel


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Re: [Xen-devel] Grant table version

[Vladimir 'φ-coder/phcoder' Serbinenko]

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On 28.10.2013 19:39, Andrew Cooper wrote:
> On 28/10/13 18:06, Vladimir 'φ-coder/phcoder' Serbinenko wrote:
>> Hello, all. I'm working on pvgrub2 and my problem is following:
>> When you first use grant tables you commit yourself to a particular
>> grant table version. GRUB has to read from disk and hence use grant
>> tables. By doing so it commits anything that it loads to the same version.
>> Would it be possible to have a hypercall to reset grant table version?
>> Of course, before doing such a call one would need to revoke all grants
>> or this call would discard all current grants.
> 
> You can change back and forth with grant table versions using the
> GNTTABOP_set_version hypercall, so long as you have no active grants.
> 
> It is not possible to revoke a grant, as the grant ABI guarantees that a
> successful grant stays mapped until explicitly unmapped.  Otherwise, a
> reset would cause unexpected pagefaults in the mapper domain.
> 
> For compatibility reasons it is not strictly required, but it is
> certainly expected that a new kernel explicitly chooses a gnttab version
> using the hypercall.  There is certainly an area for problems if pvgrub2
> uses grant v2, then leaves v2 active and hands off to an older kernel
> who expects v1 and doesn't explicitly set the version.
> 
> From this point of view, the safe course of action is to use whichever
> type of grants you want, then tear down all the front/back pairs, and
> reset the version to v1 before handing off.
> 
Thank you, that's what I did now.
> ~Andrew
> 
>>
>>
>>
>> _______________________________________________
>> Xen-devel mailing list
>> Xen-devel@lists.xen.org
>> http://lists.xen.org/xen-devel
> 



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Re: [Xen-devel] Grant table version

[Ian Campbell]

On Wed, 2013-10-30 at 13:26 +0100, Vladimir 'φ-coder/phcoder' Serbinenko
wrote:

> > From this point of view, the safe course of action is to use whichever
> > type of grants you want, then tear down all the front/back pairs, and
> > reset the version to v1 before handing off.
> > 
> Thank you, that's what I did now.

FWIW I don't think there is any need for pvgrub to be using the sorts of
advanced features enabled by gnttab v2. Even Linux doesn't use most
(any?) of them ;-)

Ian.

Re: [Xen-devel] Grant table version

[Matt Wilson]

On Thu, Oct 31, 2013 at 03:48:23PM +0000, Ian Campbell wrote:
> On Wed, 2013-10-30 at 13:26 +0100, Vladimir 'φ-coder/phcoder' Serbinenko
> wrote:
> 
> > > From this point of view, the safe course of action is to use whichever
> > > type of grants you want, then tear down all the front/back pairs, and
> > > reset the version to v1 before handing off.
> > > 
> > Thank you, that's what I did now.
> 
> FWIW I don't think there is any need for pvgrub to be using the sorts of
> advanced features enabled by gnttab v2. Even Linux doesn't use most
> (any?) of them ;-)

Indeed. Vladimir, for maximum compatibility with various versions of
Xen, it would probably be best to use gnttab v1 only.

--msw