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Re: hypercalls with 64-bit results

  • To: Juergen Gross <jgross@xxxxxxxx>
  • From: Jan Beulich <jbeulich@xxxxxxxx>
  • Date: Thu, 17 Jun 2021 10:08:19 +0200
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  • Cc: Anthony Perard <anthony.perard@xxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, George Dunlap <george.dunlap@xxxxxxxxxx>, Ian Jackson <iwj@xxxxxxxxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Julien Grall <julien@xxxxxxx>, Wei Liu <wl@xxxxxxx>, Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
  • Delivery-date: Thu, 17 Jun 2021 08:08:33 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>

On 17.06.2021 10:05, Juergen Gross wrote:
> On 17.06.21 10:00, Jan Beulich wrote:
>> On 17.06.2021 06:55, Juergen Gross wrote:
>>> On 16.06.21 18:04, Jan Beulich wrote:
>>>> Since hypercalls from the tool stack are based on ioctl(), and since
>>>> ioctl() has a return type of "int", I'm afraid there's no way we can
>>>> deal with this by adjusting function return types in the libraries.
>>>> Instead we appear to need either a new privcmd ioctl or new XENMEM_*
>>>> subops (for those cases where potentially large values get returned).
>>> I think we can just use a multicall in libxc to wrap the affected
>>> operations.
>> Hmm, we might, if we're happy for these to then not work in HVM domains
>> (PVH Dom0, which still is experimental only, or PVH/HVM DomU-s using
>> the libraries for some purpose), or if we finally wire up multicalls in
>> the HVM case (there ought to be a reason why they aren't, but I have no
>> idea what that is).
> Me neither, especially as on Arm they are supported.
> And TBH: PVH Dom0 without multicalls might be hard anyway.

Okay, let me see whether, while trying to wire them up, I run into
particular issues.




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