Re: [Xen-devel] [PATCH] x86: move vgc_flags to struct pv_vcpu

[Jan Beulich <JBeulich@xxxxxxxx>]

On 27.12.2019 12:27, Julien Grall wrote:
> Hi Jan,
> 
> On Fri, 27 Dec 2019, 09:22 Jan Beulich, <JBeulich@xxxxxxxx> wrote:
> 
>> On 23.12.2019 18:33, Julien Grall wrote:
>>> Hi Jan,
>>>
>>> On 20/12/2019 14:55, Jan Beulich wrote:
>>>> There's been effectively no use of the field for HVM.
>>>>
>>>> Also shrink the field to unsigned int, even if this doesn't immediately
>>>> yield any space benefit for the structure itself. The resulting 32-bit
>>>> padding slot can eventually be used for some other field. The change in
>>>> size makes accesses slightly more efficient though, as no REX.W prefix
>>>> is going to be needed anymore on the respective insns.
>>>>
>>>> Mirror the HVM side change here (dropping of setting the field to
>>>> VGCF_online) also to Arm, on the assumption that it was cloned like
>>>> this originally. VGCF_online really should simply and consistently be
>>>> the guest view of the inverse of VPF_down, and hence needs representing
>>>> only in the get/set vCPU context interfaces.
>>>
>>> But vPSCI is just a wrapper to get/set vCPU context interfaces. Your
>>> changes below will clearly break bring-up of secondary vCPUs on Arm.
>>>
>>> This is because arch_set_guest_info() will rely on this flag to
>>> clear/set VPF_down in the pause flags.
>>>
>>> So I think the line in arm/vpsci.c should be left alone.
>>
>> Oh, I see - I didn't notice this (ab)use despite ...
>>
> 
> Out of Interest, why do you think it is an abuse here and not in the
> toolstack?
> 
> How do you suggest to improve it? I can add it in my improvement list for
> Arm.

Oh, "abuse" was about the arch_set_guest_info() use, not the use of
the flag by the tool stack.

Jan
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