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Re: [PATCH 2/2] x86/xen: Allow per-domain usage of hardware virtualized APIC
- To: Jan Beulich <jbeulich@xxxxxxxx>
- From: Jane Malalane <Jane.Malalane@xxxxxxxxxx>
- Date: Tue, 8 Feb 2022 16:00:48 +0000
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- Thread-topic: [PATCH 2/2] x86/xen: Allow per-domain usage of hardware virtualized APIC
On 08/02/2022 13:52, Jan Beulich wrote:
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> On 08.02.2022 14:27, Jane Malalane wrote:
>> On 31/01/2022 12:05, Jan Beulich wrote:
>>> On 27.01.2022 17:01, Jane Malalane wrote:
>>>> Introduce a new per-domain creation x86 specific flag to
>>>> select whether hardware assisted virtualization should be used for
>>>> x{2}APIC.
>>>>
>>>> A per-domain option is added to xl in order to select the usage of
>>>> x{2}APIC hardware assisted vitualization, as well as a global
>>>> configuration option.
>>>>
>>>> Having all APIC interaction exit to Xen for emulation is slow and can
>>>> induce much overhead. Hardware can speed up x{2}APIC by running APIC
>>>> read/write accesses without taking a VM exit.
>>>
>>> This is odd to read for a patch which makes it possible to _turn off_
>>> acceleration. Instead it would be interesting to know what problems
>>> you have encountered making it desirable to have a way to turn this off.
>>
>> After speaking to Andrew he told me of a performance regression that was
>> reported some time ago when enabling apicv related to the pass-through
>> LAPIC timer of a HVM guest causing Xen to intercept the LAPIC timer MSR,
>> making anything that uses the LAPIC timer end up slower than it was
>> before. So, adressing your comment here, other than mentioning how being
>> able to disable acceleration for apicv can be useful when testing and
>> debugging, do you think it's worth mentioning (in more detail) that this
>> perf problem exists, in the commit message.
>
> Yes, I think it would be worth mentioning, as then the purpose of this
> change is also to be a workaround there, not just testing/debugging. In
> fact this workaround may then be viewed as the primary purpose.
>
Okay I will add this in a v3 (alongside other changes I'll have to make).
Thank you,
Jane.
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