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Re: [Xen-devel] [PATCH] xen/domctl: Drop vcpu_alloc_lock
- To: Jan Beulich <JBeulich@xxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Fri, 7 Sep 2018 10:57:22 +0100
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- Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>, Wei Liu <wei.liu2@xxxxxxxxxx>, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>, George Dunlap <George.Dunlap@xxxxxxxxxxxxx>, Tim Deegan <tim@xxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxx>, Julien Grall <julien.grall@xxxxxxx>
- Delivery-date: Fri, 07 Sep 2018 09:57:32 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Openpgp: preference=signencrypt
On 07/09/18 09:48, Jan Beulich wrote:
>>>> On 05.09.18 at 21:15, <andrew.cooper3@xxxxxxxxxx> wrote:
>> Since its introduction in c/s 8cbb5278e "x86/AMD: Add support for AMD's OSVW
>> feature in guests", the OSVW data has been corrected to be per-domain rather
>> than per-vcpu, and is initialised during XEN_DOMCTL_createdomain.
>>
>> Furthermore, because XENPF_microcode_update uses hypercall continuations to
>> move between CPUs, it drops the vcpu_alloc_lock mid update, meaning that it
>> didn't provided the interlock guarantee that the OSVW patch was looking for
>> in
>> the first place.
>>
>> This interlock serves no purpose, so take the opportunity to drop it and
>> remove a global spinlock from the hypervisor.
> I see you've rushed the patch in (perhaps to avoid objections, given
> that you've proposed this removal before, and I didn't really like it),
> so I guess we need to take it from there now.
There was nothing deliberate here. TBH, I thought the patch had been
pending on the list for longer than it had. Either way, as we are
starting the conversation again...
> The interlock didn't work as intended, I agree, but "serves no purpose"
> is wrong imo.
At the moment, I stand my by statement, because as far as I can tell,
the interlock literally does nothing.
> Rather than blindly dropping the logic, I'd have expected
> for it to be fixed: Despite the movement into XEN_DOMCTL_createdomain
> there's still a race between ucode updates and domain creation.
What race? What have I overlooked?
~Andrew
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