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Re: [PATCH v3 00/13] xen: drop hypercall function tables


  • To: Juergen Gross <jgross@xxxxxxxx>
  • From: Jan Beulich <jbeulich@xxxxxxxx>
  • Date: Tue, 8 Mar 2022 14:52:54 +0100
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  • Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>, Julien Grall <julien@xxxxxxx>, Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>, Bertrand Marquis <bertrand.marquis@xxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, George Dunlap <george.dunlap@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, Christopher Clark <christopher.w.clark@xxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
  • Delivery-date: Tue, 08 Mar 2022 13:53:06 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>

On 08.03.2022 14:44, Juergen Gross wrote:
> On 08.03.22 14:42, Jan Beulich wrote:
>> On 08.03.2022 13:56, Juergen Gross wrote:
>>> On 08.03.22 13:50, Jan Beulich wrote:
>>>> On 08.03.2022 09:39, Juergen Gross wrote:
>>>>> On 08.03.22 09:34, Jan Beulich wrote:
>>>>>> On 08.12.2021 16:55, Juergen Gross wrote:
>>>>>>> In order to avoid indirect function calls on the hypercall path as
>>>>>>> much as possible this series is removing the hypercall function tables
>>>>>>> and is replacing the hypercall handler calls via the function array
>>>>>>> by automatically generated call macros.
>>>>>>>
>>>>>>> Another by-product of generating the call macros is the automatic
>>>>>>> generating of the hypercall handler prototypes from the same data base
>>>>>>> which is used to generate the macros.
>>>>>>>
>>>>>>> This has the additional advantage of using type safe calls of the
>>>>>>> handlers and to ensure related handler (e.g. PV and HVM ones) share
>>>>>>> the same prototypes.
>>>>>>>
>>>>>>> A very brief performance test (parallel build of the Xen hypervisor
>>>>>>> in a 6 vcpu guest) showed a very slim improvement (less than 1%) of
>>>>>>> the performance with the patches applied. The test was performed using
>>>>>>> a PV and a PVH guest.
>>>>>>>
>>>>>>> Changes in V2:
>>>>>>> - new patches 6, 14, 15
>>>>>>> - patch 7: support hypercall priorities for faster code
>>>>>>> - comments addressed
>>>>>>>
>>>>>>> Changes in V3:
>>>>>>> - patches 1 and 4 removed as already applied
>>>>>>> - comments addressed
>>>>>>>
>>>>>>> Juergen Gross (13):
>>>>>>>      xen: move do_vcpu_op() to arch specific code
>>>>>>>      xen: harmonize return types of hypercall handlers
>>>>>>>      xen: don't include asm/hypercall.h from C sources
>>>>>>>      xen: include compat/platform.h from hypercall.h
>>>>>>>      xen: generate hypercall interface related code
>>>>>>>      xen: use generated prototypes for hypercall handlers
>>>>>>>      x86/pv-shim: don't modify hypercall table
>>>>>>>      xen/x86: don't use hypercall table for calling compat hypercalls
>>>>>>>      xen/x86: call hypercall handlers via generated macro
>>>>>>>      xen/arm: call hypercall handlers via generated macro
>>>>>>>      xen/x86: add hypercall performance counters for hvm, correct pv
>>>>>>>      xen: drop calls_to_multicall performance counter
>>>>>>>      tools/xenperf: update hypercall names
>>>>>>
>>>>>> As it's pretty certain now that parts of this which didn't go in yet will
>>>>>> need re-basing, I'm going to drop this from my waiting-to-be-acked 
>>>>>> folder,
>>>>>> expecting a v4 instead.
>>>>>
>>>>> Yes, I was planning to spin that up soon.
>>>>>
>>>>> The main remaining question is whether we want to switch the return type
>>>>> of all hypercalls (or at least the ones common to all archs) not
>>>>> requiring to return 64-bit values to "int", as Julien requested.
>>>>
>>>> After walking through the earlier discussion (Jürgen - thanks for the link)
>>>> I'm inclined to say that if Arm wants their return values limited to 32 
>>>> bits
>>>> (with exceptions where needed), so be it. But on x86 I'd rather not see us
>>>> change this aspect. Of course I'd much prefer if architectures didn't
>>>> diverge in this regard, yet then again Arm has already diverged in avoiding
>>>> the compat layer (in this case I view the divergence as helpful, though, as
>>>> it avoids unnecessary headache).
>>>
>>> How to handle this in common code then? Have a hypercall_ret_t type
>>> (exact naming TBD) which is defined as long on x86 and int on Arm?
>>> Or use long in the handlers and check the value on Arm side to be a
>>> valid 32-bit signed int (this would be cumbersome for the exceptions,
>>> though)?
>>
>> I was thinking along the lines of hypercall_ret_t, yes, but the
>> compiler wouldn't be helping with spotting truncation issues (we can't
>> reasonably enable the respective warnings, as they would trigger all
>> over the place). If we were to go that route, we'd rely on an initial
>> audit and subsequent patch review to spot issues. Therefore,
>> cumbersome or not, the checking approach may be the more viable one.
>>
>> Then again Julien may have a better plan in mind; I'd anyway expect
>> him to supply details on how he thinks such a transition could be done
>> safely, as he was the one to request limiting to 32 bits.
> 
> In order to have some progress I could just leave the Arm side alone
> in my series. It could be added later if a solution has been agreed
> on.
> 
> What do you think?

I see no issue with this if there's no other dependency on Arm following
suit.

Jan




 


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