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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 17:26:50 +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 15:27:06 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>

On 17.06.2021 10:08, Jan Beulich wrote:
> 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.

And, at the risk of stating the obvious, this isn't going to help 32-bit
Dom0 at all, so (assuming the immediate issue wasn't going to be taken
care of at the ovmf side) would help only one of the two tests preventing
a push right now. (I have a patch set drafted, but it's yet to be tested.
I did find though that the tool stack already uses multicalls in one
special case.)




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