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

  • To: Julien Grall <julien.grall@xxxxxxxxx>
  • From: Jan Beulich <JBeulich@xxxxxxxx>
  • Date: Fri, 27 Dec 2019 12:14:54 +0000
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  • Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>, Julien Grall <julien@xxxxxxx>, Wei Liu <wl@xxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Volodymyr Babchuk <volodymyr_babchuk@xxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
  • Delivery-date: Fri, 27 Dec 2019 12:21:45 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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  • Thread-topic: [Xen-devel] [PATCH] x86: move vgc_flags to struct pv_vcpu

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.

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