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

[Julien Grall <julien.grall@xxxxxxxxx>]

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.

Cheers,

-- 

Julien Grall

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