Re: [Xen-devel] Grant table version

[Vladimir 'Ï-coder/phcoder' Serbinenko <phcoder@xxxxxxxxx>]

On 28.10.2013 19:39, Andrew Cooper wrote:
> On 28/10/13 18:06, Vladimir 'Ï-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
> 
>>
>>
>>
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>

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