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

From: Julien Grall <julien@xen.org>
To: Jan Beulich <JBeulich@suse.com>, Julien Grall <julien.grall@gmail.com>
Cc: "Stefano Stabellini" <sstabellini@kernel.org>,
	"Wei Liu" <wl@xen.org>,
	"Andrew Cooper" <andrew.cooper3@citrix.com>,
	"xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>,
	"Volodymyr Babchuk" <volodymyr_babchuk@epam.com>,
	"Roger Pau Monné" <roger.pau@citrix.com>
Subject: Re: [Xen-devel] [PATCH] x86: move vgc_flags to struct pv_vcpu
Date: Fri, 3 Jan 2020 10:56:41 +0000	[thread overview]
Message-ID: <28da91bd-006f-79b1-38d0-e22eae4986ff@xen.org> (raw)
In-Reply-To: <86f54aec-1120-62e4-dde3-4d49c54a8188@suse.com>

Hi,

On 27/12/2019 12:14, Jan Beulich wrote:
> On 27.12.2019 12:27, Julien Grall wrote:
>> Hi Jan,
>>
>> On Fri, 27 Dec 2019, 09:22 Jan Beulich, <JBeulich@suse.com> 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.

I may have read incorrectly your e-mail, although I think my questions 
about why this is an abuse and how do you suggest to improve are still 
relevant.

Cheers,

-- 
Julien Grall

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