linux-kernel - Re: [RFC 0/6] vDSO support for Hyper-V guest on ARM64

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Date:   Fri, 24 Jan 2020 10:24:44 +0000
From:   Vincenzo Frascino <vincenzo.frascino@....com>
To:     Boqun Feng <boqun.feng@...il.com>
Cc:     linux-hyperv@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, Sasha Levin <sashal@...nel.org>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Michael Kelley <mikelley@...rosoft.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        xen-devel@...ts.xenproject.org,
        Thomas Gleixner <tglx@...utronix.de>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Will Deacon <will@...nel.org>
Subject: Re: [RFC 0/6] vDSO support for Hyper-V guest on ARM64

Hi Boqun Feng,

On 24/01/2020 06:32, Boqun Feng wrote:
> Hi Vincenzo,
> 

[...]

>>
>> I had a look to your patches and overall, I could not understand why we can't
>> use the arch_timer to do the same things you are doing with the one you
>> introduced in this series. What confuses me is that KVM works just fine with the
>> arch_timer which was designed with virtualization in mind. Why do we need
>> another one? Could you please explain?
>>
> 
> Please note that the guest VM on Hyper-V for ARM64 doesn't use
> arch_timer as the clocksource. See:
> 
> 	https://lore.kernel.org/linux-arm-kernel/1570129355-16005-7-git-send-email-mikelley@microsoft.com/
> 
> ,  ACPI_SIG_GTDT is used for setting up Hyper-V synthetic clocksource
> and other initialization work.
>

I had a look a look at it and my question stands, why do we need another timer
on arm64?

> So just to be clear, your suggestion is
> 
> 1) Hyper-V guest on ARM64 should use arch_timer as clocksource and vDSO
> will just work.
> 
> or
> 
> 2) Even though arch_timer is not used as the clocksource, we can still
> use it for vDSO.
> 
> ?
> 

Option #1 would be the preferred solution, unless there is a good reason against.

> Regards,
> Boqun
> 

-- 
Regards,
Vincenzo

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